[2.11.x][NEXUS-6790] Copy yum metadata to group in case of a single yum member

https://issues.sonatype.org/browse/NEXUS-6790
http://bamboo.s/browse/NX-OSSF335
Could a Yum group repository ever have no members? If so then we need an extra check here before we attempt to grab the zero-th member of the group.