[Bugfix] stream failure when model name not in audio endpoints (#36679)

Signed-off-by: Ekagra Ranjan <3116519+ekagra-ranjan@users.noreply.github.com>

vllm/entrypoints/openai/speech_to_text/speech_to_text.py

@@ -389,6 +389,9 @@ class OpenAISpeechToText(OpenAIServing):
         if error_check_ret is not None:
             return error_check_ret
 
+        if not request.model:
+            request.model = self.models.model_name()
+
         # If the engine is dead, raise the engine's DEAD_ERROR.
         # This is required for the streaming case, where we return a
         # success status before we actually start generating text :).