fix sdxl_lwp empty neg_prompt error issue (#4743)

* fix sdxl_lwp empty neg_prompt error issue

* fix sdxl_lwp empty neg_prompt error issue, update code format

---------
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

examples/community/lpw_stable_diffusion_xl.py

@@ -1088,25 +1088,9 @@ class SDXLLongPromptWeightingPipeline(DiffusionPipeline, FromSingleFileMixin, Lo
 
         # 3. Encode input prompt
         (cross_attention_kwargs.get("scale", None) if cross_attention_kwargs is not None else None)
-        # (
-        #     prompt_embeds,
-        #     negative_prompt_embeds,
-        #     pooled_prompt_embeds,
-        #     negative_pooled_prompt_embeds,
-        # ) = self.encode_prompt(
-        #     prompt=prompt,
-        #     prompt_2=prompt_2,
-        #     device=device,
-        #     num_images_per_prompt=num_images_per_prompt,
-        #     do_classifier_free_guidance=do_classifier_free_guidance,
-        #     negative_prompt=negative_prompt,
-        #     negative_prompt_2=negative_prompt_2,
-        #     prompt_embeds=prompt_embeds,
-        #     negative_prompt_embeds=negative_prompt_embeds,
-        #     pooled_prompt_embeds=pooled_prompt_embeds,
-        #     negative_pooled_prompt_embeds=negative_pooled_prompt_embeds,
-        #     lora_scale=text_encoder_lora_scale,
-        # )
+
+        negative_prompt = negative_prompt if negative_prompt is not None else ""
+
         (
             prompt_embeds,
             negative_prompt_embeds,