Only print warning if Ant fileset of classes to scan is empty (like Maven)

```
If no classes are in the given fileset, the Ant task should simply print a 
warning instead of failing the build.

This allows multi-module builds to succeed that share one forbidden-apis target.
```

Original issue reported on code.google.com by `uwe.h.schindler` on 14 Sep 2014 at 7:56
