Removing null validation because a null check is not needed

[sergioescala]

Removing null validation because a null check is not needed before using instanceof

What is the purpose of the change

XXXXX

Brief changelog

XXXXX

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #3020 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #3020      +/-   ##
=========================================
+ Coverage   64.28%   64.3%   +0.01%     
=========================================
  Files         584     584              
  Lines       26056   26056              
  Branches     4562    4562              
=========================================
+ Hits        16751   16756       +5     
+ Misses       7123    7120       -3     
+ Partials     2182    2180       -2

Impacted Files	Coverage Δ
...protocol/dubbo/status/ThreadPoolStatusChecker.java	0% <0%> (ø)	⬆️
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	⬇️
.../remoting/transport/netty4/NettyClientHandler.java	75% <0%> (-11.12%)	⬇️
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	⬇️
.../src/main/java/org/apache/dubbo/rpc/RpcStatus.java	67.77% <0%> (-2.23%)	⬇️
.../apache/dubbo/remoting/transport/AbstractPeer.java	60.86% <0%> (-2.18%)	⬇️
...rg/apache/dubbo/common/timer/HashedWheelTimer.java	60.62% <0%> (-1.75%)	⬇️
...pache/dubbo/registry/support/AbstractRegistry.java	80.07% <0%> (-1.51%)	⬇️
.../java/org/apache/dubbo/config/ReferenceConfig.java	56.01% <0%> (-0.38%)	⬇️
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	66.66% <0%> (+0.83%)	⬆️
... and 9 more

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[carryxyh]

LGTM.
Thx for your contribution!