fix(feed): 修复 At 内容不存在的情况下类型校验错误

Author: medz

packages/slimkit-plus-feed/src/API2/FeedController.php

@@ -469,7 +469,7 @@ public function store(StoreFeedPostRequest $request)
             return response()->json(['message' => '发布成功', 'id' => $feed->id])->setStatusCode(201);
         });
 
-        $this->sendAtMessage($feed->feed_content, $user, $feed);
+        $this->sendAtMessage((string) $feed->feed_content, $user, $feed);
 
         return $response;
     }