support json encode null and isError

[rickey17]

Motivation:

解决一个issue问题 https://github.com/sofastack/sofa-rpc/issues/1260

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

[codecov[bot]]

Codecov Report

Merging #1264 (79a6f45) into master (dabc7b8) will decrease coverage by 0.05%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1264      +/-   ##
============================================
- Coverage     72.14%   72.09%   -0.06%     
  Complexity      780      780              
============================================
  Files           411      411              
  Lines         17418    17418              
  Branches       2714     2714              
============================================
- Hits          12567    12558       -9     
- Misses         3466     3472       +6     
- Partials       1385     1388       +3     

Impacted Files	Coverage Δ
...ipay/sofa/rpc/codec/jackson/JacksonSerializer.java	86.71% <100.00%> (+0.69%)	:arrow_up:
...java/com/alipay/sofa/rpc/module/LookoutModule.java	52.38% <0.00%> (-33.34%)	:arrow_down:
...n/java/com/alipay/sofa/rpc/log/TimeWaitLogger.java	80.00% <0.00%> (-4.00%)	:arrow_down:
...com/alipay/sofa/rpc/context/RpcRuntimeContext.java	87.65% <0.00%> (-2.47%)	:arrow_down:
...pay/sofa/rpc/transport/ClientTransportFactory.java	76.92% <0.00%> (-1.54%)	:arrow_down:
...lipay/sofa/rpc/message/AbstractResponseFuture.java	56.14% <0.00%> (-0.88%)	:arrow_down:
...ansport/bolt/ReuseBoltClientConnectionManager.java	72.46% <0.00%> (+2.89%)	:arrow_up:

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