Avoid subtract overflow in filter3x3 on zero-dimension images

[SAY-5]

filter3x3 reads its dimensions and then loops 1..width - 1 / 1..height - 1. On an image with zero width or height those subtractions underflow and panic with "attempt to subtract with overflow" (reported in #3026).

Such an image has no interior pixels to filter, so this returns the empty output buffer early, matching how resize already handles the empty case. Added a regression test covering both zero-width and zero-height inputs.

Fixes #3026.

[RunDevelopment]

Thanks for the PR. I think this might be a duplicate of #3055 or #2874.

[SAY-5]

You're right, this overlaps #3055 and the abyss-policy fix in #2874 supersedes it. Closing in favor of #2874 since that addresses the root cause rather than just guarding the underflow. Thanks!