Fixed failure when target/classes does not exist when runnin mvn test phase

Fixed failure when target/classes does not exist when runnin mvn test phase

Thanks to Stephen Johnson <stejohns@redhat.com>

Details from the original email:
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The following patch stops builds failing when target/classes (or no main java exists), and target/test-classes and src/tests exist.

So for example calling

mvn test -> invokes compiler:compile and if you have export bound to process-classes phase in executions it will fail.  Maybe better to give info and carry on.  Say for example they want to leave the executions in place that deal with process-classes and also process-test-classes but they do not want it to fail if there is no java to annotate in src/classes.  The other way would be to comment out the executions bound to process-classes.  What about export being bound to process-class by default?  Could this also cause issues?

In either case I think the plugin code did checks for src/classes directory existing, in which case even call "mvn test" would fail as src/classes would not exist as no java existed in src/main only in src/test.  Have a look through the patch and see if its of any use.