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.

This NPE problem was introduced in 2.7, so backport needed.

Issue
https://issues.sonatype.org/browse/NEXUS-6230

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF26