I suspect that the Community ♦︎︎ user only bumps questions which "have at least one non-deleted answer scoring 0" because:
- the bumping is targeted at trying to move questions that the community has been interested enough to try and answer off of the Unanswered list
- a question with no answers suggests its asker has not continued to try and edit/garden it until it attracts an answerer and so, if it does not have the community showing any interest in it via upvoting and editing, the question is left in the path of the Roomba
- it would be inefficient to have a question that no one is interested enough to answer, or to improve into an answerable state, left to keep cycling through hoping to attract an answer during the next bump because:
- a question with a 0 votes answer only needs a single upvote during the bump to come off the Unanswered list
- a question with no answers needs much more volunteered effort to first get an answer written (especially if the asker thinks their job is done), and then also an upvote before it comes off the Unanswered list.