ant task silently ignores empty excludeFilter

[Crydust]

Logging a warning will prevent confusion.

Also see question on StackOverflow:
FindBugs Exclude filter with Ant
http://stackoverflow.com/questions/24275252/findbugs-exclude-filter-with-ant/35222903

[Crydust]

We make it explicit in the warning whether the file is empty or doesn't exist.

I'd like to point out that file.length() == 0 doesn't imply file.exists().
The documentation for the file.length method is here:
https://docs.oracle.com/javase/8/docs/api/java/io/File.html#length--

[Crydust]

Is there anything wrong with this pull request? I'd be happy to make changes if requested.

[iloveeclipse]

Thanks. Ping help sometimes :-)