[videodb]: keep empty series if hideemptyseries=true

As of now (advanced setting) `videolibrary.hideemptyseries` has been quite useless as empty TV shows get always deleted when cleaning the library.

I do think it makes sense to keep empty TV shows based on that setting, otherwise there's no much to hide...

I delete episodes after I watch them, this causes the TV show to be deleted from the database and re-scraped every week. On devices like a Raspberry Pi this operation is not really _instant_.

I've been using this for months and I've been a more happy person since then...