Use empty query instead of SELECT 1 to check active connection

Extracted from #220055 (comment 463405732)

ActiveRecord uses SELECT 1 to check the liveness of a connection. We are seeing this query causes a very high frequency on GitLab.com.

As shown in #220055 (comment 463405732), using an empty query ; instead of SELECT 1 increases throughput - which is what this issue is about.

We can see if we can patch this locally and if successful, upstream this into Rails.