category: fix category search in explore

This is a change that replicates the fix done via PR #6093.
This is based over v5.1.1 as the fix seems necessary for
the category search to work properly on the explore screen.

Author: sivaraam

app/src/main/java/fr/free/nrw/commons/category/CategoryClient.kt

@@ -124,7 +124,7 @@ class CategoryClient
                 }.map {
                     it
                         .filter { page ->
-                            !page.categoryInfo().isHidden
+                            page.categoryInfo() == null || !page.categoryInfo().isHidden
                         }.map {
                             CategoryItem(
                                 it.title().replace(CATEGORY_PREFIX, ""),

app/src/main/java/fr/free/nrw/commons/wikidata/mwapi/MwQueryPage.java

@@ -36,7 +36,7 @@ public class MwQueryPage extends BaseModel {
         return title;
     }
 
-    @NonNull public CategoryInfo categoryInfo() {
+    public CategoryInfo categoryInfo() {
         return categoryinfo;
     }