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

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

vllm/model_executor/models/chameleon.py

@@ -987,11 +987,11 @@ class ChameleonForConditionalGeneration(nn.Module, SupportsMultiModal,
     def get_language_model(self) -> torch.nn.Module:
         return self.model
 
-    def get_multimodal_embeddings(
-            self, **kwargs: object) -> Optional[MultiModalEmbeddings]:
+    def get_multimodal_embeddings(self,
+                                  **kwargs: object) -> MultiModalEmbeddings:
         image_input = self._parse_and_validate_image_input(**kwargs)
         if image_input is None:
-            return None
+            return []
         assert self.model.vqmodel is not None
         image_tokens = self.model.get_image_tokens(image_input["data"].to(
             self.config.torch_dtype))