在json中丢失字段时无法更新数据库

我正在制作一个api,可以帮助用户按照输入数据更新他们的信息.但是当输入json有字段"password"时它会成功更新但是当json没有这个字段时我无法更新数据库中的数据.这是我用来更新数据的代码:

public function updateUserInfo(Request $request){
        $postData = $request->all();
        $data = json_decode($postData['postData'], true);
        if(isset($data['password'])){
            $data['password'] = bcrypt($data['password']);
        }
        $popData = $data['UserId'];
        unset($data['UserId']);
        $updateInfo = array();
        foreach($data as $info){
            if($info != null){
                $updateInfo[] = $info;
            }
        }
        $result =  DB::update(GeneralFunctions::makeUpdateString($data, 'User', ' UserId = '.$popData), $updateInfo);
        if($result != null && $result == true){
            return response()->json(['success'=>true, 'data'=>'Update Successful']);
        }else{
            return response()->json(['success'=>false, 'error'=>'We have encountered an error, try again later!']);
        }
    }

当一切正常时,这是json:

$postData = '{ "UserId" : "1",   "password":"12345", "UserName": "minhkhang",  "Address": "11/200" }'

这是json会导致错误,因为它缺少密码字段:

$postData = '{ "UserId" : "1", "UserName": "minhkhang",  "Address": "11/200" }'

这是我用来使更新字符串跟随输入json的代码:

 public static function makeUpdateString($keyvalarr, $table, $where){
        $stringSQL = 'UPDATE '.$table. ' SET ' ;
        foreach($keyvalarr as $fieldname => $updateval){
            if($updateval != null){
                $stringSQL .= $fieldname.' = ? , ';
            }
        }
        $stringSQL =  substr($stringSQL, 0, -2);
        if($where != null){
            $stringSQL .= 'WHERE '.$where;
        }
        return $stringSQL;
    }

谢谢.