refactor: avoid NPE caused by children field data problem (#2645)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.0

#### What this PR does / why we need it:
菜单和分类数列表加强健壮性，防止因为 children 字段存在垃圾数据时导致 NPE 问题
#### Which issue(s) this PR fixes:
a part of #2643

#### Special notes for your reviewer:
how to test it?
1. 创建一个层级结构菜单，比如
```
A
|-B
|-C
```
2. 删除菜单 C 然后访问主题端首页不会发生以下异常信息
```
Caused by: java.lang.NullPointerException: Cannot invoke "run.halo.app.theme.finders.vo.MenuItemVo.setParentName(String)" because "childNode" is null
```

/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
修复获取菜单和分类列表时会出现 NPE 的问题
```

src/main/java/run/halo/app/theme/finders/impl/CategoryFinderImpl.java

@@ -82,9 +82,12 @@ public class CategoryFinderImpl implements CategoryFinder {
 
         nameIdentityMap.forEach((name, value) -> {
             List<String> children = value.getSpec().getChildren();
-            if (children != null) {
+            if (children == null) {
+                return;
+            }
             for (String child : children) {
                 CategoryTreeVo childNode = nameIdentityMap.get(child);
+                if (childNode != null) {
                     childNode.setParentName(name);
                 }
             }

src/main/java/run/halo/app/theme/finders/impl/MenuFinderImpl.java

@@ -127,9 +127,12 @@ public class MenuFinderImpl implements MenuFinder {
 
         nameIdentityMap.forEach((name, value) -> {
             LinkedHashSet<String> children = value.getSpec().getChildren();
-            if (children != null) {
+            if (children == null) {
+                return;
+            }
             for (String child : children) {
                 MenuItemVo childNode = nameIdentityMap.get(child);
+                if (childNode != null) {
                     childNode.setParentName(name);
                 }
             }

src/test/java/run/halo/app/theme/finders/impl/CategoryFinderImplTest.java

@@ -226,6 +226,7 @@ class CategoryFinderImplTest {
     }
 
     private List<Category> moreCategories() {
+        // see also https://github.com/halo-dev/halo/issues/2643
         String s = """
             [
                {
@@ -318,7 +319,8 @@ class CategoryFinderImplTest {
                         "bd95f914-22fc-4de5-afcc-a9ffba2f6401",
                         "e1150fd9-4512-453c-9186-f8de9c156c3d",
                         "acf09686-d5a7-4227-ba8c-3aeff063f12f",
-                        "ed064d5e-2b6f-4123-8114-78d0c6f2c4e2"
+                        "ed064d5e-2b6f-4123-8114-78d0c6f2c4e2",
+                        "non-existent-children-name"
                      ]
                   },
                   "status":{

src/test/java/run/halo/app/theme/finders/impl/MenuFinderImplTest.java

@@ -90,7 +90,7 @@ class MenuFinderImplTest {
         Menu menuX = menu("X", of("G"));
         Menu menuY = menu("Y", of("F"));
 
-        MenuItem itemE = menuItem("E", of("A", "C"));
+        MenuItem itemE = menuItem("E", of("A", "C", "non-existent-children-name"));
         MenuItem itemG = menuItem("G", null);
         MenuItem itemF = menuItem("F", of("H"));
         MenuItem itemA = menuItem("A", of("B"));