PERF: Switch ActiveRecord PG connection active check to use empty query. (PR #13323)

See Use an empty query instead of SELECT 1 by engwan · Pull Request #42368 · rails/rails · GitHub

The impact is not quantifiable at the time of this writing but prelimary investigation shows that SELECT 1 accounts for 0.09 of CPU time on a database. Note that Discourse runs thousands of databases so the small impact may be amplified by the large number of databases that we run.