dillon1000/react

[Fizz] Don't flush empty segments (#24054)

Before this change, we would sometimes write segments without any content
in them. For example for a Suspense boundary that immediately suspends
we might emit something like:

<div hidden id="123">
  <template id="456"></template>
</div>

Where the outer div is just a temporary wrapper and the inner one is a
placeholder for something to be added later.

This serves no purpose.

We should ideally have a heuristic that holds back segments based on byte
size and time. However, this is a straight forward clear win for now.