Chgrp verbose prints unmapped gid

[cerdelen]

chgrp with verbosity flag prints an empty string instead of the gid.

fix #9915

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/shuf/shuf-reservoir is no longer failing!
Congrats! The gnu test tests/sort/sort-stale-thread-mem is no longer failing!
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[cerdelen]

I am a littlebit confused about the output in the tests. They print 'retained as runner' or i also see 'retained as wheel'.

I only changed that instead of not printing anything in case of a gid being unmapped the gid will be printed, so before in these cases nothing shouldve been printed. Why where they not red before.