Cleanup: Wrong return value

`drop_target_tooltip` return type was changed to std::string in
7ca4dcac5a

Pull Request: https://projects.blender.org/blender/blender/pulls/118361

source/blender/editors/interface/interface_drop.cc

@@ -55,13 +55,13 @@ std::string drop_target_tooltip(const ARegion &region,
 {
   const char *disabled_hint_dummy = nullptr;
   if (!drop_target.can_drop(drag, &disabled_hint_dummy)) {
-    return nullptr;
+    return {};
   }
 
   const std::optional<DropLocation> drop_location = drop_target.choose_drop_location(region,
                                                                                      event);
   if (!drop_location) {
-    return nullptr;
+    return {};
   }
 
   const DragInfo drag_info{drag, event, *drop_location};