Fix the problem of not being able to rebuild search index (#6323)

#### What type of PR is this?

/kind bug
/area core
/milestone 2.18.x

#### What this PR does / why we need it:

This PR fixes the problem of not being able to rebuild search index due to null description and updateTimestamp.

#### Which issue(s) this PR fixes:

Fixes #6317 

#### Does this PR introduce a user-facing change?

```release-note
修复无法正常刷新搜索引擎的问题
```

application/src/main/java/run/halo/app/search/lucene/LuceneSearchEngine.java

@@ -194,8 +194,11 @@ public class LuceneSearchEngine implements SearchEngine, InitializingBean, Dispo
                 }
 
                 var description = doc.get("description");
-                var hlDescription =
-                    highlighter.getBestFragment(this.analyzer, "description", description);
+                String hlDescription = null;
+                if (description != null) {
+                    hlDescription =
+                        highlighter.getBestFragment(this.analyzer, "description", description);
+                }
 
                 var content = doc.get("content");
                 var hlContent = highlighter.getBestFragment(this.analyzer, "content", content);
@@ -310,7 +313,9 @@ public class LuceneSearchEngine implements SearchEngine, InitializingBean, Dispo
             }
 
             doc.add(new TextField("title", haloDoc.getTitle(), YES));
-            doc.add(new TextField("description", haloDoc.getDescription(), YES));
+            if (haloDoc.getDescription() != null) {
+                doc.add(new TextField("description", haloDoc.getDescription(), YES));
+            }
             doc.add(new TextField("content", haloDoc.getContent(), YES));
             doc.add(new StringField("recycled", Boolean.toString(haloDoc.isRecycled()), YES));
             doc.add(new StringField("exposed", Boolean.toString(haloDoc.isExposed()), YES));
@@ -335,7 +340,9 @@ public class LuceneSearchEngine implements SearchEngine, InitializingBean, Dispo
             var creationTimestamp = haloDoc.getCreationTimestamp();
             doc.add(new LongField("creationTimestamp", creationTimestamp.toEpochMilli(), YES));
             var updateTimestamp = haloDoc.getUpdateTimestamp();
-            doc.add(new LongField("updateTimestamp", updateTimestamp.toEpochMilli(), YES));
+            if (updateTimestamp != null) {
+                doc.add(new LongField("updateTimestamp", updateTimestamp.toEpochMilli(), YES));
+            }
             doc.add(new StringField("permalink", haloDoc.getPermalink(), YES));
             return doc;
         }
@@ -375,8 +382,11 @@ public class LuceneSearchEngine implements SearchEngine, InitializingBean, Dispo
 
             var creationTimestamp = doc.getField("creationTimestamp").numericValue().longValue();
             haloDoc.setCreationTimestamp(Instant.ofEpochMilli(creationTimestamp));
-            var updateTimestamp = doc.getField("updateTimestamp").numericValue().longValue();
-            haloDoc.setUpdateTimestamp(Instant.ofEpochMilli(updateTimestamp));
+            var updateTimestampField = doc.getField("updateTimestamp");
+            if (updateTimestampField != null) {
+                var updateTimestamp = updateTimestampField.numericValue().longValue();
+                haloDoc.setUpdateTimestamp(Instant.ofEpochMilli(updateTimestamp));
+            }
             // handle content later
             return haloDoc;
         }

application/src/main/java/run/halo/app/search/post/PostHaloDocumentsProvider.java

@@ -2,6 +2,7 @@ package run.halo.app.search.post;
 
 import org.springframework.stereotype.Component;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import run.halo.app.content.ContentWrapper;
 import run.halo.app.content.PostService;
 import run.halo.app.core.extension.content.Post;
@@ -36,6 +37,11 @@ public class PostHaloDocumentsProvider implements HaloDocumentsProvider {
         // get content
         return paginatedOperator.list(Post.class, options)
             .flatMap(post -> postService.getReleaseContent(post)
+                .switchIfEmpty(Mono.fromSupplier(() -> ContentWrapper.builder()
+                    .content("")
+                    .raw("")
+                    .rawType("")
+                    .build()))
                 .map(contentWrapper -> convert(post, contentWrapper))
             );
     }

application/src/test/java/run/halo/app/search/lucene/LuceneSearchEngineTest.java

@@ -2,6 +2,7 @@ package run.halo.app.search.lucene;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.assertArg;
@@ -160,7 +161,7 @@ class LuceneSearchEngineTest {
         var gotHaloDoc = result.getHits().get(0);
         assertEquals("fake-id", gotHaloDoc.getId());
         assertEquals("<fake-tag>fake</fake-tag>-title", gotHaloDoc.getTitle());
-        assertEquals("<fake-tag>fake</fake-tag>-desc", gotHaloDoc.getDescription());
+        assertNull(gotHaloDoc.getDescription());
         assertEquals("<fake-tag>fake</fake-tag>-content", gotHaloDoc.getContent());
     }
 
@@ -169,13 +170,13 @@ class LuceneSearchEngineTest {
         haloDoc.setId("fake-id");
         haloDoc.setMetadataName("fake-name");
         haloDoc.setTitle("fake-title");
-        haloDoc.setDescription("fake-desc");
+        haloDoc.setDescription(null);
         haloDoc.setContent("fake-content");
         haloDoc.setType("fake-type");
         haloDoc.setOwnerName("fake-owner");
         var now = Instant.now();
         haloDoc.setCreationTimestamp(now);
-        haloDoc.setUpdateTimestamp(now);
+        haloDoc.setUpdateTimestamp(null);
         haloDoc.setPermalink("/fake-permalink");
         haloDoc.setAnnotations(Map.of("fake-anno-key", "fake-anno-value"));
         return haloDoc;

application/src/test/java/run/halo/app/search/post/PostHaloDocumentsProviderTest.java

@@ -56,7 +56,23 @@ class PostHaloDocumentsProviderTest {
                 assertEquals("fake-content", doc.getContent());
             })
             .verifyComplete();
+    }
 
+    @Test
+    void shouldFetchAllIfNoContent() {
+        var post = createFakePost();
+        when(paginatedOperator.list(same(Post.class), any(ListOptions.class)))
+            .thenReturn(Flux.just(post));
+        when(postService.getReleaseContent(post)).thenReturn(Mono.empty());
+        provider.fetchAll()
+            .as(StepVerifier::create)
+            .assertNext(doc -> {
+                assertEquals("post.content.halo.run", doc.getType());
+                assertEquals("fake-post", doc.getMetadataName());
+                assertEquals("post.content.halo.run-fake-post", doc.getId());
+                assertEquals("", doc.getContent());
+            })
+            .verifyComplete();
     }
 
     Post createFakePost() {