Optimize sampling for empty images

[HeroicKatora]

This is a tip of an iceberg of better sampling, but the most critical case. Up-sampling in general may in the implementation allocate a larger temporary buffer than its input. Of course this makes little semantic sense here: after all, the actual information can not increase by this.

If one dimension increases while the other decreases the unfortunate consequence is that callers may somewhat reasonably expect a small buffer but internally will get a very large buffer. The approach of swapping sampling orders accordingly (first down, then up) might address the memory issue but is lossy.

So instead let's fix the most pressing issue: if no information was present in the input, nothing can be lost and we can pretend to perform everything by a very small intermediate image.

closes: #2340

[fintelia]

I think it would be cleaner to add a check at the start of resize to immediately return ImageBuffer::new(nwidth, nheight) if any new or old image dimensions are empty. vertical_sample is only called from resize and blur. In the blur case, height==new_height so there isn't much difference from this change. And in resize, it seems nicer to handle all combinations of input and output sizes being empty at once.