Allow None extents in C setimage()

[radarhere]

Follow up from #9395

[hugovk]

Could this be considered a breaking change? Although it's fixing unusual and perhaps undocumented behaviour.

Might be worth adding a brief note in the release notes? We could also wait for the Pillow 13.0.0 release in October, but that's probably unnecessary?

[radarhere]

#9395 changed the C setimage() so extents needed to be passed in. However, I've realised that both before and after that, the C setimage() never accepted None.

For background, #1938 added the Python setimage(), which accepted None for the extents, and tested for it accordingly. #8285 added None as a possible value to the extents type hint in a tile.

I think we should aim for the same behaviour with the C and Python setimage(). It is fair enough to say that removing None as a possible extents value for Python setimage() is a breaking change, so I'm now going in the opposite direction with this PR - allowing None as an extents value in the C setimage().