Geometry race condition fix

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

konova/forms/geometry_form.py

@@ -63,6 +63,7 @@ class SimpleGeomForm(BaseForm):
         geom = self.data["geom"]
         if geom is None or len(geom) == 0:
             # empty geometry is a valid geometry
+            self.cleaned_data["geom"] = MultiPolygon(srid=DEFAULT_SRID_RLP).ewkt
             return is_valid
         geom = json.loads(geom)
 
@@ -106,6 +107,8 @@ class SimpleGeomForm(BaseForm):
                 return is_valid
 
             features.append(polygon)
+
+        # Unionize all geometry features into one new MultiPolygon
         form_geom = MultiPolygon(srid=DEFAULT_SRID_RLP)
         for feature in features:
             form_geom = form_geom.union(feature)