100% abstain votes in map voting now chooses a random map
Closes #2443 (closed)
This allows for voting for abstain when alone in a server to go to random votable map.
As a bonus it fixes the first votable option always being picked when sv_vote_gametype_default_current
== 1 instead of a random map even though someone voted for abstain. With no (abstain) votes it'll still go for first.