Return a 1x1 image when bbox is smaller than a single pixel

[JackDunnNZ]

This was originally motivated by hitting a zero-division error when calling Reader.feature with a feature that happened to be smaller than one pixel in the image:

File "site-packages/rasterio/transform.py", line 189, in from_bounds
    (east - west) / width, (south - north) / height)
    ~~~~~~~~~~~~^~~~~~~~~
ZeroDivisionError: float division by zero

I traced it back to the result of part being a 0x0 image when the bbox was smaller than a single pixel of the image, which this change fixes and returns a 1x1 image instead.

I'm not 100% sure about this, but it seems to make sense to me that part returning a single pixel is the correct outcome for this case, as this is still the valid image data for the bbox being requested.

[vincentsarago]

🙏 that totally makes senses

thank you

[JackDunnNZ]

Thanks!