• mxqd_control: Check prerequisites against empty group tags

We want to be able to use group tags as a stigma so that servers can be
configured to accept only jobs without certain tags. For example,
mxqi can add a tag "mxqi" to its jobs and the servers which shouldn't
run mxqi jobs can use "!mxqi" as their prerequisites.

However, for that to work, we need to evaluate server prerequistes even
for jobs which don't have any tags.

Remove the check, which skips server prerequistes evaluation for jobs
without tags.

• mxqsub: Remove a stray space character from output
• mxqd_control: Check prerequisites against empty group tags