[3.2]Fix:triple protocol echoService invoke.

[wuwen5]

What is the purpose of the change?

The triple protocol encountered an error during echo testing (EchoService)

Exception in thread "main" java.lang.NullPointerException
	at org.apache.dubbo.rpc.protocol.tri.TripleInvoker.isSync(TripleInvoker.java:196)
	at org.apache.dubbo.rpc.protocol.tri.TripleInvoker.doInvoke(TripleInvoker.java:155)

fix #15128

Checklist

• Make sure there is a GitHub_issue field for the change.
• 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. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[heliang666s]

Thanks for your contribution~However, You should run mvn spotless:apply to format before commit your code.@wuwen5

[wuwen5]

@heliang666s Done. Thanks!