[10.1.0 regression] Palette images save as blank PDFs

[BertrandBordage]

Minimal example (you can use this tiny PNG for example):

from PIL import Image

image = Image.open('test.png')
image = image.convert('P')
image.save('test.pdf')

Output PDF with Pillow 10.0.1:

Output PDF with Pillow 10.1.0:

Issue faced with Python 3.11.6 on Ubuntu 22.04 and Debian 12 (bookworm). I also had the same issue in Docker environments, so I could make a Docker image if needed.

[radarhere]

Testing, I find this was caused by #7289.

I've created PR #7555 to resolve this.

[BertrandBordage]

So quick, incredible job @radarhere!