NEXUS-6731: GA level metadata is not created on upload

if generated POM used and there is no main artifact, only
classified artifact.
NEXUS-6731: UT ensuring that MD maintenance happens without classifier passed over
Merge pull request #656 from sonatype/nexus-2.9.x-NEXUS-6731-ga-level-metadata-on-upload

NEXUS-6731: GA level metadata is not created on upload