[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.

packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js

@@ -495,6 +495,10 @@ describe('ReactDOMFizzServer', () => {
       pipe(writable);
     });
     expect(getVisibleChildren(container)).toEqual(<div>Loading...</div>);
+    // Because there is no content inside the Suspense boundary that could've
+    // been written, we expect to not see any additional partial data flushed
+    // yet.
+    expect(container.firstChild.nextSibling).toBe(null);
     await act(async () => {
       resolveElement({default: <Text text="Hello" />});
     });

packages/react-server/src/ReactFizzServer.js

@@ -488,7 +488,7 @@ function renderSuspenseBoundary(
     // We use the safe form because we don't handle suspending here. Only error handling.
     renderNode(request, task, content);
     contentRootSegment.status = COMPLETED;
-    newBoundary.completedSegments.push(contentRootSegment);
+    queueCompletedSegment(newBoundary, contentRootSegment);
     if (newBoundary.pendingTasks === 0) {
       // This must have been the last segment we were waiting on. This boundary is now complete.
       // Therefore we won't need the fallback. We early return so that we don't have to create
@@ -1430,6 +1430,29 @@ function abortTask(task: Task): void {
   }
 }
 
+function queueCompletedSegment(
+  boundary: SuspenseBoundary,
+  segment: Segment,
+): void {
+  if (
+    segment.chunks.length === 0 &&
+    segment.children.length === 1 &&
+    segment.children[0].boundary === null
+  ) {
+    // This is an empty segment. There's nothing to write, so we can instead transfer the ID
+    // to the child. That way any existing references point to the child.
+    const childSegment = segment.children[0];
+    childSegment.id = segment.id;
+    childSegment.parentFlushed = true;
+    if (childSegment.status === COMPLETED) {
+      queueCompletedSegment(boundary, childSegment);
+    }
+  } else {
+    const completedSegments = boundary.completedSegments;
+    completedSegments.push(segment);
+  }
+}
+
 function finishedTask(
   request: Request,
   boundary: Root | SuspenseBoundary,
@@ -1463,7 +1486,7 @@ function finishedTask(
         // If it is a segment that was aborted, we'll write other content instead so we don't need
         // to emit it.
         if (segment.status === COMPLETED) {
-          boundary.completedSegments.push(segment);
+          queueCompletedSegment(boundary, segment);
         }
       }
       if (boundary.parentFlushed) {
@@ -1482,8 +1505,8 @@ function finishedTask(
         // If it is a segment that was aborted, we'll write other content instead so we don't need
         // to emit it.
         if (segment.status === COMPLETED) {
+          queueCompletedSegment(boundary, segment);
           const completedSegments = boundary.completedSegments;
-          completedSegments.push(segment);
           if (completedSegments.length === 1) {
             // This is the first time since we last flushed that we completed anything.
             // We can schedule this boundary to emit its partially completed segments early