src: fixup node_platform blocking task drain

https://github.com/nodejs/node/pull/15428 was supposed to account
for upcoming changes in V8 upstream, but while addressing review
comments a bug was introduced; `DrainBackgroundTasks()` should
always at least perform one blocking drain on the background task
queue.

PR-URL: https://github.com/nodejs/node/pull/15639
Refs: https://github.com/nodejs/node/pull/15428
Refs: f27b5e4b


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>