Rails 6.1 introduced the following behavior in ActiveRecord:

Avoid making query when using `where(attr: [])`
https://github.com/rails/rails/pull/37266

Mongoid should implement something similar. For example, the following queries are guaranteed never to return any results:

Users.any_in(name: [])
Users.where(name: 'Bob').where(name: 'Joe')

Such code should return a Mongoid::Criteria but calling .to_a on that criteria returns [] without generating a DB query.