Handle empty results.

Author: WaVEV

django_mongodb/compiler.py

@@ -28,7 +28,7 @@ def execute_sql(
         try:
             query = self.build_query(columns)
         except EmptyResultSet:
-            return None
+            return iter([]) if result_type == MULTI else None
 
         return (
             (self._make_result(row, columns) for row in query.fetch())