Always return voters_count in polls

According to Mastodon docs, this field is null if the poll is single-choice. I asked gargron about this and he said that this was probably added later, but the docs say both were added at the same time, so I guess it was only added later during development. He said that this field is always set in Mastodon.

I changed the schema and our behavior to always return the voters_count to make all of this less weird.