What does this PR do?

Fixes #13377

This PR fixes a bug in QwenImagePipeline and QwenImageEditPipeline where Classifier-Free Guidance (CFG) is incorrectly disabled when users manually pass negative_prompt_embeds alongside a None mask.

Root cause:
The encode_prompt method naturally optimizes an all-ones prompt_embeds_mask by converting and returning it as None. However, the has_neg_prompt condition in the __call__ method strictly required negative_prompt_embeds_mask is not None to activate CFG. This logical mismatch caused do_true_cfg to evaluate to False even when valid negative embeddings were provided.

Changes:

• Relaxed the has_neg_prompt condition.
• It now correctly evaluates has_neg_prompt to True as long as negative_prompt_embeds is not None, since a None mask simply implies that no padding/masking is required.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? [Bug] QwenImagePipeline silently disables CFG when passing negative_prompt_embeds if mask is None (which encode_prompt returns by default) #13377.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@asomoza @yiyixuxu