SONARFBUGS-8 Findbugs analysis should not fail if only package-info.java files are analysed.

This is the same as https://github.com/SonarSource/sonar-java/pull/12, for the new findbugs repository.
I just wanted to make sure the previous PR still works. It does.

Intercepts the use case where only package-info.java files (which do not generate any class to be analyzed by Findbugs) are present in the source folders.