ActiveRecord::Relation#blank? should `LIMIT 1`

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!

Bonus performance improvements will also happen for `#present?`, which
merely calls the negation of `#blank?`

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #10539 from davidcelis/ar-sql-improvements

Improve performance of ActiveRecord::Relation#blank?
Revert "Merge pull request #10539 from davidcelis/ar-sql-improvements"

This reverts commit 257fa6897d9c85da16b7c9fcb4ae3008198d320e, reversing
changes made to 94725b81f5588e4b0f43222c4f142c3135941b4b.

The build failed

https://travis-ci.org/rails/rails/builds/7883546
Update Getting Started Guide - Strong Parameters
	modified:   guides/source/getting_started.md
Merge pull request #10872 from AJAlabs/master

Update Getting Started Guide - Strong Parameter [ci skip]