Fix T85027: Crash dragging a collection over empty Outliner

Filtering may make the Outliner tree empty. The function to find the drop
element should just return null then and let the caller decide how to deal with
it.

source/blender/editors/space_outliner/outliner_dragdrop.c

@@ -140,6 +140,11 @@ static TreeElement *outliner_drop_insert_find(bContext *C,
   TreeElement *te_hovered;
   float view_mval[2];
 
+  /* Emtpy tree, e.g. while filtered. */
+  if (BLI_listbase_is_empty(&space_outliner->tree)) {
+    return NULL;
+  }
+
   UI_view2d_region_to_view(
       &region->v2d, event->mval[0], event->mval[1], &view_mval[0], &view_mval[1]);
   te_hovered = outliner_find_item_at_y(space_outliner, &space_outliner->tree, view_mval[1]);