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

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

vllm/model_executor/models/phi4mm.py

@@ -1112,11 +1112,12 @@ class Phi4MMForCausalLM(nn.Module, SupportsLoRA, SupportsMultiModal):
                                            image_attention_mask)
         return image_embeds
 
-    def get_multimodal_embeddings(
-            self, **kwargs: object) -> Optional[MultiModalEmbeddings]:
+    def get_multimodal_embeddings(self,
+                                  **kwargs: object) -> MultiModalEmbeddings:
 
         modalities = self._parse_and_validate_multimodal_inputs(**kwargs)
         if not modalities:
+            return []
             return None
 
         # The result multimodal_embeddings is tuple of tensors, with each