bugfix: Compatible with Protobuf empty object.

[imangry]

Motivation:

When sending or returning a Protobuf empty object with a content length of 0,just like PagePB.Response.getDefaultInstance(), the client or server will report an error. This modification will be compatible with this situation.

03:03:00.363 [SOFA-SEV-BOLT-BIZ-12200-3-T2] ERROR c.a.s.r.c.bolt.SofaRpcSerialization - traceId=null, rpcId=null, Request deserializeContent exception, msg=Content of request is null
com.alipay.remoting.exception.DeserializationException: Content of request is null
	at com.alipay.sofa.rpc.codec.bolt.SofaRpcSerialization.deserializeContent(SofaRpcSerialization.java:273)
	at com.alipay.remoting.rpc.protocol.RpcRequestCommand.deserializeContent(RpcRequestCommand.java:148)
	at com.alipay.remoting.rpc.RpcCommand.deserialize(RpcCommand.java:117)
	at com.alipay.remoting.rpc.RpcCommand.deserialize(RpcCommand.java:138)
	at com.alipay.remoting.rpc.protocol.RpcRequestProcessor.deserializeRequestCommand(RpcRequestProcessor.java:286)
	at com.alipay.remoting.rpc.protocol.RpcRequestProcessor.doProcess(RpcRequestProcessor.java:143)
	at com.alipay.remoting.rpc.protocol.RpcRequestProcessor$ProcessTask.run(RpcRequestProcessor.java:393)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
	at java.base/java.lang.Thread.run(Thread.java:1623)

Exception in thread "main" com.alipay.sofa.rpc.core.exception.SofaRpcException: Failed to call org.apache.dubbo.benchmark.bean.UserServiceDubbo$IUserService.createUser on remote server bolt://127.0.0.1:12200, return null
	at com.alipay.sofa.rpc.client.FailoverCluster.doInvoke(FailoverCluster.java:81)
	at com.alipay.sofa.rpc.client.AbstractCluster.invoke(AbstractCluster.java:298)
	at com.alipay.sofa.rpc.client.ClientProxyInvoker.invoke(ClientProxyInvoker.java:83)

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 72.02%. Comparing base (1e99afb) to head (818b533). Report is 13 commits behind head on master.

Files	Patch %	Lines
...ipay/sofa/rpc/codec/bolt/SofaRpcSerialization.java	66.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             master    #1376    +/-   ##
==========================================
  Coverage     72.01%   72.02%            
- Complexity      784      795    +11     
==========================================
  Files           416      422     +6     
  Lines         17661    17813   +152     
  Branches       2752     2768    +16     
==========================================
+ Hits          12719    12830   +111     
- Misses         3538     3574    +36     
- Partials       1404     1409     +5     

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