final fields for pre-compiled traits aren't processed correctly

From slack:

Testing 2.4.8 by running through the Grails tests (master branch) and seeing NPE's on some tests that exercise Traits. Can be fairly quickly replicated by running `./gradlew :grails-plugin-interceptors:test`

same tests pass in 2.4.7 up thru commit https://github.com/apache/groovy/commit/c00a75a89a15e46c2afacf6e72256cd4484bd5f1 which causes the tests to fail with NPE's, don't have a lot of time to look into it, just wanted to mention it

the Trait class has a final property in it and when it's used it's null, removing the final modifier gets rid of the problem https://github.com/grails/grails-core/blob/master/grails-plugin-interceptors/src/main/groovy/grails/artefact/Interceptor.groovy#L113