pagure_bot: generate correct message when updating a ticket without votes
This fixes a small glitch that if we updated a discussion ticket which haven't
had any votes yet, we'd have the vote summary completely empty instead of saying
'Nobody voted yet'. This fixes the problem and covers the function with unit
tests as a bonus (it was completely untested before).
Merges: https://pagure.io/fedora-qa/blockerbugs/pull-request/213