NEXUS-6230: Nullcheck was missing for listedGav

As the folder might contain maven metadata and other stuff
that M2GavCalculator cannot interpret, and will return
null.
Merge pull request #321 from sonatype/NEXUS-6230-npe-snap-remover

NEXUS-6230: Nullcheck was missing for listedGav
Merge remote-tracking branch 'origin/master' into rapture