[Frontend] Use new Renderer for Completions and Tokenize API (#32863)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

examples/online_serving/prompt_embed_inference_with_openai_client.py

@@ -60,9 +60,7 @@ def main():
 
     completion = client.completions.create(
         model=model_name,
-        # NOTE: The OpenAI client does not allow `None` as an input to
-        # `prompt`. Use an empty string if you have no text prompts.
-        prompt="",
+        prompt=None,
         max_tokens=5,
         temperature=0.0,
         # NOTE: The OpenAI client allows passing in extra JSON body via the