PixArtAlphaPipeline does not handle negative_prompt list correctly

### Describe the bug

Passing negative_prompt as list to PixArtAlphaPipeline will eventually cause a "list object has no attribute lower" error.

The encode_prompt function documentation shows that it can be a list as well:
https://github.com/huggingface/diffusers/blob/7404f1e9dcdac286caf6ca3cfa43b294f61583bf/src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py#L308

However the function type hints restrict it to a str instead:
https://github.com/huggingface/diffusers/blob/7404f1e9dcdac286caf6ca3cfa43b294f61583bf/src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py#L291

These restrictions are completely arbitrary, since
a) prompt is correctly handled for the same case,
b) negative_prompt is only used in one place and there it just gets converted to a list anyway:

https://github.com/huggingface/diffusers/blob/7404f1e9dcdac286caf6ca3cfa43b294f61583bf/src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py#L392

### Reproduction

Call Pixart-Sigma with negative_prompt as list instead of a single string.

### Logs
(sorry for images)


### System Info

Windows 11
PyTorch 2.2.2 with CUDA support
Diffusers 0.28.0.dev0 @ https://github.com/huggingface/diffusers/commit/7404f1e9dcdac286caf6ca3cfa43b294f61583bf

### Who can help?

@DN6 @sayakpaul @lawrence-cj

Start in src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py, at encode_prompt and the negative_prompt handling around line 392. Reproduce PixArt-Sigma with negative_prompt as a list, then verify that the pipeline accepts the list without the reported lower error and handles it consistently with prompt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.