AngularJS(前端）和Laravel(后端）中的JSON/Arrays中为NULL

我很难处理null值。每当我向服务器发送JSON时，前端中的所有null值都会变成字符串形式的"null"。我该如何处理？

AngularJS:在我的控制器中。我正在使用ng文件上传插件从表单发送值https://github.com/danialfarid/ng-file-upload/wiki/PHP-Example

$scope.saveNewDocument = function(){
    var formAttachments = Upload.upload({
        url: appServer.baseURL + appServer.api.generalDocuments, 
        method: 'POST',
        file: $scope.formData.formAttachments,
        sendFieldsAs: 'form',
        fields: $scope.formData // $scope.formData is ng-model in view forms
    }).then(function(success){
        //some codes
    }, function(error){
        console.log(error);
    });
};

Laravel:在我的控制器中。

 public function store(Request $request)
    {
        $request = $request->all();
        //if I get values from request. All null becomes "null"
    }