Outliner: Don't show empty base elements in the library overrides mode

In the library overrides mode, in some situations there would be empty
base elements like "Collections" or "Objects". Don't show them, it's
confusing wihout use. Code just failed to consider that case.

source/blender/editors/space_outliner/tree/tree_display_override_library.cc

@@ -104,6 +104,13 @@ ListBase TreeDisplayOverrideLibrary::add_library_contents(Main &mainvar)
     }
   }
 
+  /* Remove ID base elements that turn out to be empty. */
+  LISTBASE_FOREACH_MUTABLE (TreeElement *, te, &tree) {
+    if (BLI_listbase_is_empty(&te->subtree)) {
+      outliner_free_tree_element(te, &tree);
+    }
+  }
+
   return tree;
 }