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

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

vllm/model_executor/models/qwen2_audio.py

@@ -350,11 +350,11 @@ class Qwen2AudioForConditionalGeneration(nn.Module, SupportsMultiModal,
     def get_language_model(self) -> torch.nn.Module:
         return self.language_model
 
-    def get_multimodal_embeddings(
-            self, **kwargs: object) -> Optional[MultiModalEmbeddings]:
+    def get_multimodal_embeddings(self,
+                                  **kwargs: object) -> MultiModalEmbeddings:
         audio_input = self._parse_and_validate_audio_input(**kwargs)
         if audio_input is None:
-            return None
+            return []
         masked_audio_features = self._process_audio_input(audio_input)
         return masked_audio_features