Use (0, 0) as placeholder size in stubs

[radarhere]

Within our stub plugins, we use (1, 1) as a placeholder size.

Pillow/src/PIL/BufrStubImagePlugin.py

Lines 51 to 53 in 627f3ea

	# make something up
	self._mode = "F"
	self._size = 1, 1

I think (0, 0) might more clearly indicate that there's nothing here yet.