dmstats: only generate aux data separator if group tag is defined

dmstats: only generate aux data separator if group tag is defined

Fix a bug in _stats_set_aux() that causes bogus data to appear
in the 'userdata' field of stats reports when previously grouped
regions are ungrouped:

/var/tmp/File With Spaces: Created new group with 1 region(s) as group ID 0.

Removed group ID 0 on fedora-root

Name             GrpID RgID ObjType RgStart RgSize  #Areas ArSize  ProgID  UserData
fedora-root          -    0 region    6.39g 100.00m      1 100.00m dmstats #-
                                                                           ^^
This is the aux_data separator character followed by empty user data.
The _stats_set_aux() function should only emit the separator if
there is a valid group descriptor for the region.