[Fizz] Don't bail out of flushing if we still have pending root tasks (…

…facebook#27385)

The idea for this check is that we shouldn't flush anything before we
flush the shell. That may or may not hold true in future formats like
RN.

It is a problem for resuming because with resuming it's possible to have
root tasks that are used for resuming but the shell was already flushed
so we can have completed boundaries before the shell has fully resumed.
What matters is whether the parent has already flushed or not.

It's not technically necessary to bail early because there won't be
anything in partialBoundaries or completedBoundaries because nothing
gets added there unless the parent has already flushed.

It's not exactly slow to have to check the length of three arrays so
it's probably not a big deal.

Flush partials in an early preamble needs further consideration
regardless.

packages/react-server/src/ReactFizzServer.js

@@ -3778,9 +3778,6 @@ function flushCompletedQueues(
         // We haven't flushed the root yet so we don't need to check any other branches further down
         return;
       }
-    } else if (request.pendingRootTasks > 0) {
-      // We have not yet flushed the root segment so we early return
-      return;
     }
 
     if (enableFloat) {