Geometry race condition fix

* fixes race condition for geometry conflict and parcel calculation
* harmonizes empty geometries from None/MultiPolygonEmpty to MultiPolygonEmpty

konova/models/geometry.py

@@ -116,6 +116,11 @@ class Geometry(BaseResource):
 
         """
         from konova.models import Parcel, District, ParcelIntersection, Municipal, ParcelGroup
+
+        if self.geom.empty:
+            # Nothing to do
+            return
+
         parcel_fetcher = ParcelWFSFetcher(
             geometry_id=self.id,
         )