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

tests/test_queries/test_queryset_level_methods.py

@@ -286,6 +286,9 @@ async def test_bulk_create():
         completed = await ToDo.objects.filter(completed=True).all()
         assert len(completed) == 2
 
+        with pytest.raises(ormar.exceptions.ModelListEmptyError):
+            await ToDo.objects.bulk_create([])
+
 
 @pytest.mark.asyncio
 async def test_bulk_create_json_field():