fix: provide default value for mcp_servers in application.py

--bug=1062220 --user=刘瑞斌 【应用】新建简单应用报错 https://www.tapd.cn/62980211/s/1778977

Author: liuruibin

apps/application/serializers/application.py

@@ -282,7 +282,7 @@ def to_application_model(user_id: str, workspace_id: str, application: Dict):
                 work_flow={},
                 mcp_enable=application.get('mcp_enable', False),
                 mcp_tool_ids=application.get('mcp_tool_ids', []),
-                mcp_servers=application.get('mcp_servers'),
+                mcp_servers=application.get('mcp_servers', {}),
                 mcp_source=application.get('mcp_source', 'referencing'),
                 tool_enable=application.get('tool_enable', False),
                 tool_ids=application.get('tool_ids', []),