Mappings without sources throw NullPointerException

[JasonWThompson]

This issue occurs in the master branch.

When building an RPM that includes a mapping without sources, an NullPointerException is thrown. This is due to an issue in SpecWriter.java on line 242. Specifically this line of code:

final boolean noFiles = map.getSources().isEmpty() || (scanner.isEverythingIncluded() && links.isEmpty());

map.getSources() will return null when sources are not defined and thus throws the NPE when calling isEmpty().

A couple of ways to fix this would be to either return an empty list from getSources() when Mapping.source is null or to check for null in the line above in addition to checking if sources is empty.