Previously, we'd run the sanity checks if `is_mysql` was not set to `false`. This caused problems for DB drop-ins that didn't define `is_mysql` at all. Instead, we can just check if `is_mysql` is `empty()`. Also fix some unit tests that accidently ran correctly because of the strict `false ===` comparison. Fixes #33501. git-svn-id: https://develop.svn.wordpress.org/trunk@34655 602fd350-edb4-49c9-b593-d223f7449a82