Exception raised when grabbing fails and cropping is attempted

[timmarkhuff]

If grabbing fails for whatever reason, and if there are some post-processing operations to be applied (zoom or crop), an exception will be raised because the grabbed frame is None and None cannot be zoomed or cropped.

For example, if you run:

config = {
    'options': {
        'zoom': {
            'digital':
            2.0
        }
    }
}
grabber.apply_options(config)

grabber.grab()

It will raise something like:

    grabber.apply_options(config)
                    ^^^^^^^^^^^^^^
  File "/home/pi/parkmon/pm_env/lib/python3.11/site-packages/framegrab/grabber.py", line 331, in grab
    frame = self._digital_zoom(frame)
            ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/pi/parkmon/pm_env/lib/python3.11/site-packages/framegrab/grabber.py", line 407, in _digital_zoom
    top = (frame.shape[0] - frame.shape[0] / digital_zoom) / 2

The solution to this should be just to return None in this case, which is what would happen if grabbing failed and there was no zooming or cropping.