support json encode null and isError

[rickey17]

Motivation:

Explain the context, and why you're making that change. To make others understand what is the problem you're trying to solve.

Modification:

1、支持json序列化器序列化null对象。 2、支持不管异常类型中的errorMessage是否为空，都能识别为异常

[sofastack-bot[bot]]

Hi @rickey17, welcome to SOFAStack community, Please sign Contributor License Agreement!

After you signed CLA, we will automatically sync the status of this pull request in 3 minutes.

[rickey17]

cla