support(bulk-create): add raise_error flag when bulk-create empty models (#853)

* add raise_error flag when bulk-create empty models

* if not raise_error, return

* fix ut cov

* raise exception when it's empty

ormar/queryset/queryset.py

@@ -1134,6 +1134,10 @@ class QuerySet(Generic[T]):
         :param objects: list of ormar models already initialized and ready to save.
         :type objects: List[Model]
         """
+
+        if not objects:
+            raise ModelListEmptyError("Bulk create objects are empty!")
+
         ready_objects = [obj.prepare_model_to_save(obj.dict()) for obj in objects]
 
         # don't use execute_many, as in databases it's executed in a loop