Cleanup: Use utility function to clear ListBase

source/blender/editors/space_outliner/space_outliner.cc

@@ -563,7 +563,7 @@ static void outliner_space_blend_read_data(BlendDataReader *reader, SpaceLink *s
     /* we only saved what was used */
     space_outliner->storeflag |= SO_TREESTORE_CLEANUP; /* at first draw */
   }
-  space_outliner->tree.first = space_outliner->tree.last = nullptr;
+  BLI_listbase_clear(&space_outliner->tree);
   space_outliner->runtime = nullptr;
 }