swift – 如何为alamofire Post请求中的一个参数传递一个nil值

let image: UIImage = UIImage()
    let imageData = UIImagePNGRepresentation(image)
    let base64String = imageData?.base64EncodedStringWithOptions(.Encoding64CharacterLineLength)

    let parameters = [
        "first_name": "XXXXX","email" : "1234@gmail.com","password" : "password","profile_picture" : base64String]

Alamofire.request(.POST,"http://abc/public/user/register",parameters: parameters,encoding: .JSON,headers: nil)

        .progress { bytesWritten,totalBytesWritten,totalBytesExpectedToWrite in
            print(totalBytesWritten)

            // This closure is NOT called on the main queue for performance
            // reasons. To update your ui,dispatch to the main queue.
            dispatch_async(dispatch_get_main_queue()) {
                print("Total bytes written on main queue: (totalBytesWritten)")
           }
        }
        .responseJSON { response in
            debugPrint(response)
    }

即使profile_pictures为空,响应也应该成功.我知道可以通过可选链接完成,但不知道如何继续！