[V1] Change return type on get_multimodal_embeddings() (#19446)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

vllm/model_executor/models/granite_speech.py

@@ -706,10 +706,11 @@ class GraniteSpeechForConditionalGeneration(
     def get_multimodal_embeddings(
         self,
         **kwargs: object,
-    ) -> Optional[MultiModalEmbeddings]:
+    ) -> MultiModalEmbeddings:
         """Compute the audio embeddings if audio inputs are present."""
         audio_input = self._parse_and_validate_audio_input(**kwargs)
         if audio_input is None:
+            return []
             return None
         audio_features = self._process_audio_input(audio_input)
         return audio_features