If a poll contains empty options (which is apparently possible on Pleroma),
it is created without them. However, the poll update code doesn't filter
empty options, and thus:
1. Clear known votes, as it assumes the set of options has changed
2. Errors out because it tries adding empty options, which fails validation
This commit fixes that by filtering them out the same way they are filtered
out at poll creation time.
* Add voters count to polls
* Add ActivityPub serialization and parsing of voters count
* Add support for voters count in WebUI
* Move incrementation of voters count out of redis lock
* Reword “voters” to “people”
* Process incoming poll tallies update
* Send Update on poll vote
* Do not send Updates for a poll more often than once every 3 minutes
* Include voters in people to notify of results update
* Schedule closing poll worker on poll creation
* Add new notification type for ending polls
* Add front-end support for ended poll notifications
* Fix UpdatePollSerializer
* Fix Updates not being triggered by local votes
* Fix tests failure
* Fix web push notifications for closing polls
* Minor cleanup
* Notify voters of both remote and local polls when those close
* Fix delivery of poll updates to mentioned accounts and voters
