[Model] Fix a check for None but the return value was empty list in Gemma3 MM...

[Model] Fix a check for None but the return value was empty list in Gemma3 MM vision_embeddings (#21479)
Signed-off-by: Hongmin Fan <fanhongmin@google.com>

vllm/model_executor/models/gemma3_mm.py

@@ -627,7 +627,7 @@ class Gemma3ForConditionalGeneration(nn.Module, SupportsMultiModal, SupportsPP,
 
             inputs_embeds = self.get_input_embeddings(input_ids,
                                                       vision_embeddings)
-            if vision_embeddings is not None:
+            if (vision_embeddings is not None) and len(vision_embeddings) != 0:
                 kwargs = self.prepare_attn_masks(
                     input_ids,
                     positions,