fix: enhance meta validation to check for empty dictionary

--bug=1058076 --user=刘瑞斌 【知识库】飞书知识库-文档列表操作列-设置命中处理方式为“直接回答” 报异常 https://www.tapd.cn/62980211/s/1724448

Author: liuruibin

apps/knowledge/serializers/document.py

@@ -131,7 +131,7 @@ def get_meta_valid_map():
 
     def is_valid(self, *, document: Document = None):
         super().is_valid(raise_exception=True)
-        if 'meta' in self.data and self.data.get('meta') is not None:
+        if 'meta' in self.data and self.data.get('meta') is not None and self.data.get('meta') != {}:
             knowledge_meta_valid_map = self.get_meta_valid_map()
             valid_class = knowledge_meta_valid_map.get(document.type)
             valid_class(data=self.data.get('meta')).is_valid(raise_exception=True)