Avoid a spurious deprecation warning for database URLs

When DATABASE_URL is set, configurations#[] will return a valid
configuration for any key, so we must be more explicit in our check.
This actually makes the method behave more like it was before, because
`#fetch`-with-block also doesn't use the hash fallback.

Uncovered by @guilleiguaran while investigating #14495 -- this actually
may be related to the original report, but we don't have enough info to
confirm.