[Bugfix] send None sentinel on final commit so server properly sends transcription.done (#33963)

Signed-off-by: pjs102793 <pjs102793@naver.com>
Co-authored-by: Nick Hill <nickhill123@gmail.com>
(cherry picked from commit 5458eb835d)

tests/entrypoints/openai/test_realtime_validation.py

@@ -129,5 +129,5 @@ async def test_multi_chunk_streaming(
                 " First words I spoke in the original phonograph."
                 " A little piece of practical poetry. Mary had a little lamb,"
                 " it sleeps with quite a flow, and everywhere that Mary went,"
-                " the lamb was sure to go"
+                " the lamb was sure to go."
             )