refactor: add early return for invalid document type in document_serializers.py

--bug=1057562 --user=王孝刚 【知识库】飞书知识库对接，设置命中处理方式保存报错 https://www.tapd.cn/57709429/s/1736833

Author: wxg0103

apps/dataset/serializers/document_serializers.py

@@ -141,6 +141,8 @@ def is_valid(self, *, document: Document = None):
         if 'meta' in self.data and self.data.get('meta') is not None:
             dataset_meta_valid_map = self.get_meta_valid_map()
             valid_class = dataset_meta_valid_map.get(document.type)
+            if valid_class is None:
+                return
             valid_class(data=self.data.get('meta')).is_valid(raise_exception=True)