src: fixup node_platform blocking task drain

nodejs/node#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: nodejs/node#15639
Refs: nodejs/node#15428
Refs: f27b5e4bdaafc73a830a0451ee3c641b8bcd08fe
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/node_platform.cc

@@ -86,8 +86,9 @@ static void RunForegroundTask(uv_timer_t* handle) {
 }
 
 void NodePlatform::DrainBackgroundTasks() {
-  while (FlushForegroundTasksInternal())
+  do {
     background_tasks_.BlockingDrain();
+  } while (FlushForegroundTasksInternal());
 }
 
 bool NodePlatform::FlushForegroundTasksInternal() {