Check if self.im is not None

[radarhere]

For a (0, 0) image, im.load() currently returns None.

>>> from PIL import Image
>>> im = Image.new("RGB", (1, 1))
>>> im.load()
<PixelAccess object at 0x103b865f0>
>>> im = Image.new("RGB", (0, 0))
>>> im.load()
>>> 

This is because load() tests if self.im is truthy.

Pillow/src/PIL/Image.py

Lines 867 to 876 in 92c26a7

	if self.im:
	if cffi and USE_CFFI_ACCESS:
	if self.pyaccess:
	return self.pyaccess
	from . import PyAccess
	self.pyaccess = PyAccess.new(self, self.readonly)
	if self.pyaccess:
	return self.pyaccess
	return self.im.pixel_access(self.readonly)

And the image size is what is used to determine the boolean value.

>>> from PIL import Image
>>> im = Image.new("RGB", (10, 10))
>>> len(im.im)
100
>>> bool(im.im)
True
>>> im = Image.new("RGB", (0, 0))
>>> len(im.im)
0
>>> bool(im.im)
False

So this PR instead changes the Python code to check self.im is not None, there and in other locations. I've added a check to assert im.load() is not None for a (0, 0) image.

This then required existing checks in Tests/test_image_access.py on (0, 0) images to be updated. Why? Because once the (0, 0) image started returning an access object from load(), it is revealed that PixelAccess and PyAccess return different errors when the index is out of range.

Pillow/src/_imaging.c

Line 449 in 92c26a7

PyErr_SetString(PyExc_IndexError, outside_image);

Pillow/src/PIL/PyAccess.py

Line 126 in 92c26a7

raise ValueError("pixel location out of range")