Assert palette is not None

[radarhere]

This is part of https://github.com/python-pillow/Pillow/pull/8362 - I'm hoping to break down that PR into easier-to-review chunks.

Image's palette may be None

>>> from PIL import Image
>>> im = Image.new("RGB", (1, 1))
>>> im.palette is None
True

So before using it, let's assert that it isn't None.