Handle zero dimensions in ImagingOffset

[Mengxi-Xu]

Changes proposed in this pull request:

• Fixed division-by-zero crash in ImagingOffset when image width or height is zero
• Added dimension checks before performing xoffset %= im->xsize and yoffset %= im->ysize

Reference:
This PR addresses the division-by-zero issue previously reported in security advisory GHSA-f87v-9578-p47w. The fix follows the recommended approach: checking image dimensions before modulo operations and returning an empty image copy early when either dimension is zero.

[hugovk]

• Added unit tests for ImageChops.offset() with empty images, e.g., (0, 1) and (1, 0)

This is missing from the PR.

[Mengxi-Xu]

• Added unit tests for ImageChops.offset() with empty images, e.g., (0, 1) and (1, 0)

This is missing from the PR.

Yes, thank you for the reminder. I have rewritten the commit description.

[hugovk]

Thank you! 🎉