Slightly improve code for annotation.py

cvat/apps/engine/annotation.py

@@ -845,9 +845,9 @@ def merge(self, objects, start_frame, overlap):
 
         # Nothing to merge here. Just add all int_objects if any.
         if not old_objects_by_frame or not int_objects_by_frame:
-            for old_obj in [item for sublist in old_objects_by_frame.values() for item in sublist]:
-                self._modify_unmached_object(old_obj,
-                    start_frame + overlap)
+            for frame in old_objects_by_frame:
+                for old_obj in old_objects_by_frame[frame]:
+                    self._modify_unmached_object(old_obj, start_frame + overlap)
             self.objects.extend(int_objects)
             return
 
@@ -952,7 +952,7 @@ def _calc_polygons_similarity(p0, p1):
             return overlap_area / (p0.area + p1.area - overlap_area)
 
         has_same_type  = obj0["type"] == obj1["type"]
-        has_same_label = obj0["label_id"] == obj1["label_id"] if "label_id" in obj0 else True
+        has_same_label = obj0.get("label_id") == obj1.get("label_id")
         if has_same_type and has_same_label:
             if obj0["type"] == models.ShapeType.RECTANGLE:
                 p0 = geometry.box(*obj0["points"])