[FIXED JENKINS-24110] - Explicitly handle null Executables in hudson.model.Executor

This change improves the handling of errors in hudson.model.Executor, which cause issues like JENKINS-18164.
The change also introduces several annotations in order to prevent further issues.

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
[FIXED JENKINS-24309] Caching class loads and misses from UberClassLoader.
Merge branch 'JENKINS-24110' of github.com:synopsys-arc-oss/jenkins
[JENKINS-24110] Noting merge of #1352.