Allow disabled logging when JAVA_HOME is not known

[jwgmeligmeyling]

getLogger() may return null, this is problematic for occasions where logging is used without a null check. This PR adds the required null check for two cases in the JavacCompiler.

[olamy]

I wonder how getLogger() can return null?
what is your use case? does it happen within a maven plugin?

[jwgmeligmeyling]

My use case is Arquillian Shrinkwrap! Unfortunately they do not attach a logger (I am creating a PR for that too, but mean while I think it would be wise you consistently use logging, either a null return of getLogger() should be treated everywhere, or the null should not be accepted in the first place.

[jwgmeligmeyling]

Thanks! 👍