Minor code refactoring

[ralf0131]

What is the purpose of the change

Minor code refactoring, no functional change. All of them are reported by IntelliJ IDEA.

Brief changelog

• remove unnecessary null check
• fix typo
• code simplification
• etc.

Verifying this change

It should have no functional change.

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in test module.
• Run mvn clean install -DskipTests & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-io]

Codecov Report

Merging #2428 into master will decrease coverage by 0.08%.
The diff coverage is 78.94%.

@@             Coverage Diff              @@
##             master    #2428      +/-   ##
============================================
- Coverage     55.14%   55.06%   -0.09%     
+ Complexity     5274     5268       -6     
============================================
  Files           573      573              
  Lines         25459    25460       +1     
  Branches       4537     4533       -4     
============================================
- Hits          14039    14019      -20     
- Misses         9327     9348      +21     
  Partials       2093     2093

Impacted Files	Coverage Δ	Complexity Δ
...rpc/cluster/configurator/AbstractConfigurator.java	43.47% <0%> (+0.92%)	8 <1> (ø)	⬇️
...bo/rpc/cluster/support/FailoverClusterInvoker.java	88.88% <100%> (+4.44%)	7 <0> (ø)	⬇️
...bo/rpc/cluster/support/FailbackClusterInvoker.java	86.48% <100%> (-0.36%)	8 <0> (ø)
...bo/rpc/cluster/support/AbstractClusterInvoker.java	70.87% <100%> (-0.29%)	42 <1> (ø)
.../java/org/apache/dubbo/config/ReferenceConfig.java	51.07% <100%> (-0.35%)	42 <0> (-1)
...bbo/rpc/cluster/support/ForkingClusterInvoker.java	85.71% <66.66%> (ø)	7 <0> (ø)	⬇️
...bo/rpc/cluster/support/FailfastClusterInvoker.java	80% <85.71%> (+13.33%)	4 <0> (ø)	⬇️
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	7% <0%> (-1%)
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-10.17%)	12% <0%> (-1%)
...apache/dubbo/rpc/protocol/dubbo/FutureAdapter.java	58.06% <0%> (-6.46%)	3% <0%> (ø)
... and 7 more

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[diecui1202]

It looks good to me.