[Bugfix] Fix Ovis Image crash when guidance_scale is set without negative_prompt (#1956)

Signed-off-by: Dnoob <dxpouo@gmail.com>

vllm_omni/diffusion/models/ovis_image/pipeline_ovis_image.py

@@ -685,6 +685,7 @@ class OvisImagePipeline(nn.Module, CFGParallelMixin, DiffusionPipelineProfilerMi
 
         negative_text_ids = None
         if do_classifier_free_guidance:
+            negative_prompt = negative_prompt if negative_prompt is not None else ""
             negative_prompt_embeds, negative_text_ids = self.encode_prompt(
                 prompt=negative_prompt,
                 prompt_embeds=negative_prompt_embeds,