[Dubbo-619] Fix consumer will generate wrong stackTrace

[zhaixiaoxiang]

Fix #619
Recur this bug in https://github.com/zhaixiaoxiang/dubbo-examples/tree/master/hessian-npe-demo

What is the purpose of the change

Fix Consumer will generate wrong stackTrace when provider throws exception with empty stackTrace. Please see https://www.yuque.com/fa902k/id5z6r/sr041v

Brief changelog

update:
org.apache.dubbo.rpc.RpcResult

add:
org.apache.dubbo.rpc.RpcResultTest

Verifying this change

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[codecov-io]

Codecov Report

Merging #2956 into master will increase coverage by 0.1%.
The diff coverage is 90%.

@@            Coverage Diff            @@
##           master    #2956     +/-   ##
=========================================
+ Coverage   63.94%   64.04%   +0.1%     
=========================================
  Files         584      584             
  Lines       26056    26066     +10     
  Branches     4563     4565      +2     
=========================================
+ Hits        16661    16695     +34     
+ Misses       7212     7188     -24     
  Partials     2183     2183

Impacted Files	Coverage Δ
.../src/main/java/org/apache/dubbo/rpc/RpcResult.java	96.87% <90%> (-3.13%)	⬇️
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	⬇️
.../remoting/transport/netty4/NettyClientHandler.java	75% <0%> (-11.12%)	⬇️
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	⬇️
...rg/apache/dubbo/common/timer/HashedWheelTimer.java	62.02% <0%> (-0.35%)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	68.45% <0%> (ø)	⬆️
...a/org/apache/dubbo/monitor/dubbo/DubboMonitor.java	89.71% <0%> (+1.86%)	⬆️
...org/apache/dubbo/rpc/protocol/AbstractInvoker.java	66.12% <0%> (+3.22%)	⬆️
...e/dubbo/remoting/transport/netty/NettyChannel.java	62.35% <0%> (+4.7%)	⬆️
...onfig/spring/extension/SpringExtensionFactory.java	84.61% <0%> (+5.12%)	⬆️
... and 5 more

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