always include validators.Optional in form validators for fields that are a list of references

flask_admin/contrib/mongoengine/form.py

@@ -114,7 +114,7 @@ class CustomModelConverter(orm.ModelConverter):
                 return AjaxSelectMultipleField(loader, **kwargs)
 
             kwargs['widget'] = form.Select2Widget(multiple=True)
-            kwargs.setdefault('validators', [validators.Optional()])
+            kwargs.setdefault('validators', []).append(validators.Optional())
 
             # TODO: Support AJAX multi-select
             doc_type = field.field.document_type