Improve performance of ActiveRecord::Relation#blank?

This is an SQL improvement to ActiveRecord::Relation#blank?. Currently,
it calls `to_a` on the Relation, which loads all records in the
association, and calls `blank?` on the loaded Array. There are other
ways, however, to check the emptiness of an association that are far
more performant. `#empty?`, `#exists?` and `#any?` all attach a `LIMIT
1` to the SQL query before firing it off, which is a nice query
improvement. `#blank?` should do the same! This is easily done by
making `#blank?` an alias of `#empty?`.

Bonus performance improvements will also happen for `#present?`, which
merely calls the negation of `#blank?`

Signed-off-by: David Celis <me@davidcel.is>