Improve type checking in tests

[radarhere]

This is part of #8362 - I'm hoping to break down that PR into easier-to-review chunks.

In our test suite, this PR

• asserts that a tuple is returned by getpixel(), rather than float or None, before index access is used

Pillow/src/PIL/Image.py

Lines 1680 to 1682 in 0e3f51d

	def getpixel(
	self, xy: tuple[int, int] | list[int]
	) -> float | tuple[int, ...] | None:

• asserts that load() does not return None before pixel access is used

Pillow/src/PIL/Image.py

Line 882 in 0e3f51d

def load(self) -> core.PixelAccess | None:

• assert that palette is not None before trying to access one of its properties

Pillow/src/PIL/Image.py

Line 555 in 0e3f51d

self.palette: ImagePalette.ImagePalette | None = None