Avoid npe in netty 4.1 instrumentation

[laurit]

Resolves #5901

[pkgonan]

@mateuszrzeszutek @trask
Hi, when is the release date of 1.13.1 ?

[trask]

hi @pkgonan! we are releasing 1.13.1 soon, but this fix isn't included because it doesn't meet our criteria for patching:

In general, patch releases are only made for regressions, security vulnerabilities, memory leaks and deadlocks.

if you are looking for the latest SNAPSHOT build (which includes the fix), check out https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/CONTRIBUTING.md#snapshot-builds

1.14.0 (which will have this fix) should be released ~3rd week of May