Fix empty listing of single page (#2474)

#### What type of PR is this?

/kind bug
/area core
/milestone 2.0

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

Fix empty listing of single page while using PostgreSQL.

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

```release-note
None
```

src/main/java/run/halo/app/content/impl/SinglePageServiceImpl.java

@@ -140,32 +140,38 @@ public class SinglePageServiceImpl implements SinglePageService {
 
     private Mono<ListedSinglePage> getListedSinglePage(SinglePage singlePage) {
         Assert.notNull(singlePage, "The singlePage must not be null.");
+        return Mono.just(singlePage)
+            .map(sp -> {
                 ListedSinglePage listedSinglePage = new ListedSinglePage();
                 listedSinglePage.setPage(singlePage);
-        return Mono.just(listedSinglePage)
-            .flatMap(page -> listContributors(singlePage.getStatusOrDefault().getContributors())
-                .map(contributors -> {
-                    page.setContributors(contributors);
-                    return page;
-                }));
+                return listedSinglePage;
+            })
+            .flatMap(lsp ->
+                setContributors(singlePage.getStatusOrDefault().getContributors(), lsp));
     }
 
-    private Mono<List<Contributor>> listContributors(List<String> usernames) {
+    private Mono<ListedSinglePage> setContributors(List<String> usernames,
+        ListedSinglePage singlePage) {
+        return listContributors(usernames)
+            .collectList()
+            .doOnNext(singlePage::setContributors)
+            .map(contributors -> singlePage)
+            .defaultIfEmpty(singlePage);
+    }
+
+    private Flux<Contributor> listContributors(List<String> usernames) {
         if (usernames == null) {
-            return Mono.empty();
+            return Flux.empty();
         }
         return Flux.fromIterable(usernames)
-            .map(username -> client.fetch(User.class, username)
+            .flatMap(username -> client.fetch(User.class, username))
             .map(user -> {
                 Contributor contributor = new Contributor();
-                    contributor.setName(username);
+                contributor.setName(user.getMetadata().getName());
                 contributor.setDisplayName(user.getSpec().getDisplayName());
                 contributor.setAvatar(user.getSpec().getAvatar());
                 return contributor;
-                })
-            )
-            .flatMap(Function.identity())
-            .collectList();
+            });
     }
 
     boolean contains(Collection<String> left, List<String> right) {

src/main/java/run/halo/app/core/extension/reconciler/SinglePageReconciler.java

@@ -172,7 +172,8 @@ public class SinglePageReconciler implements Reconciler<Reconciler.Request> {
 
             if (excerpt.getAutoGenerate()) {
                 contentService.getContent(spec.getHeadSnapshot())
-                    .subscribe(content -> {
+                    .blockOptional()
+                    .ifPresent(content -> {
                         String contentRevised = content.content();
                         status.setExcerpt(getExcerpt(contentRevised));
                     });
@@ -184,7 +185,7 @@ public class SinglePageReconciler implements Reconciler<Reconciler.Request> {
             String headSnapshot = singlePage.getSpec().getHeadSnapshot();
             contentService.listSnapshots(Snapshot.SubjectRef.of(SinglePage.KIND, name))
                 .collectList()
-                .subscribe(snapshots -> {
+                .blockOptional().ifPresent(snapshots -> {
                     List<String> contributors = snapshots.stream()
                         .map(snapshot -> {
                             Set<String> usernames = snapshot.getSpec().getContributors();