Users: Use null instead of empty string to indicate missing UID/GID

In attempt to fix #156, an empty string was stored in the description
instead of numeric UID/GID in case a numeric value couldn't be parsed
from /etc/passwd. This was an ugly hack and it unnecessarily made
UID/GID a multi-type value. A cleaner solution is to use "null".
Groups: Use null instead of empty string to indicate missing GID

In attempt to fix #156, an empty string was stored in the description
instead of numeric GID in case a numeric value couldn't be parsed from
/etc/group. This was an ugly hack and it unnecessarily made GID a
multi-type value. A cleaner solution is to use "null".
UsersInspector: Improve comment
Merge pull request #174 from SUSE/users-groups-uid-gid-null

[Review] Request from 'dmajda' @ 'SUSE/machinery/users-groups-uid-gid-null'