xcode 10，当 json 里边有 null 时，赋值会崩溃

HandyJSON 版本：4.2.0-beta1
我定义了一个继承自 NSObject, HandyJSON, NSCoding 的类型，当后台返回的 json 中某个字段的值是 null 时，会崩溃在类 ExtendCustomModelType 的 66 行。此时，convertedValue = nil。
定义的类型：
```
class GFUserInfo: NSObject, HandyJSON, NSCoding {

var sIM: String?
var state: String?
var cityCode: String?
var companyName: String?
var companyNameState: Int = 0
var headImg: String?
var nickName: String?
var userCertificationSwitch: Int = 0
var vcardState: Int = 0
var vlicenseState: Int = 0

var lastRemindTime: TimeInterval = 0
}
```

json：
```
"userInfo": {
"userID": "",
"sIM": "",
"nickName": "WHY",
"headImg": null,
"cardImgF": "",
"cardImgB": "",
"licenseO": "",
"licenseC": "",
"verifyCard": "1",
"verifyLicense": "1",
"verifyCredit": "1",
"verifyDeposit": "0"
}
```

同样的的代码，在 xcode 9，HandyJSON 版本 4.1.3 不会崩溃。

Start at ExtendCustomModelType line 66 and reproduce the crash with the GFUserInfo model and the JSON containing a null headImg value. Compare the behavior with HandyJSON 4.1.3 and Xcode 9; done means the same null input no longer crashes under Xcode 10 and the affected mapping behavior is covered by a regression test.

Written by the indexing model from the issue text.