optimize: fast fail when channel is null

[lyl2008dsg]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

This PR enhances the RmNettyRemotingClient#sendRegisterMessage method by improving its error handling when the channel parameter is null.

Ⅱ. Does this pull request fix one issue?

Fixes #7057

Ⅲ. Why don't you add test cases (unit test/integration test)?

org.apache.seata.core.rpc.netty.RmNettyClientTest#testSendAsyncRequestWithNullChannelLogsWarning

Ⅳ. Describe how to verify it

1. Run Unit Tests: Execute the test suite to ensure all new and existing tests pass.

   mvn test
   

• org.apache.seata.core.rpc.netty.RmNettyClientTest#testSendAsyncRequestWithNullChannelLogsWarning

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.65%. Comparing base (340ea92) to head (01d776d).
Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x    #7075   +/-   ##
=========================================
  Coverage     52.65%   52.65%           
- Complexity     6665     6666    +1     
=========================================
  Files          1129     1129           
  Lines         40168    40168           
  Branches       4708     4708           
=========================================
+ Hits          21150    21152    +2     
  Misses        16999    16999           
+ Partials       2019     2017    -2     

Files with missing lines	Coverage Δ
...ta/core/rpc/netty/AbstractNettyRemotingClient.java	30.27% <100.00%> (+1.37%)	⬆️

... and 4 files with indirect coverage changes

[funky-eyes]

LGTM