Improve rendering performance (#64408)
This PR improves the server rendering performance a bit, especially for
the App Router. It has mainly 2 changes.
A rough benchmark test with a 300kB Lorem Ipsum page, SSR is about 18%
faster.
### Improve `getServerInsertedHTML`
- Avoid an extra `renderToReadableStream` if we already know the content
will be empty
- Avoid `await stream.allReady` for better parallelism with
`streamToString()`
- Increase the `progressiveChunkSize`
### Improve `createHeadInsertionTransformStream`
- Only do chunk splitting and enqueuing if the inserted string is not
empty
