Catch parted returning None as the searched partition

Certain USB devices aren't handled correctly by
python-parted. As soon as the USB devices are
filled/rewritten with zeros (dd if=/dev/zero of=/dev/sdX)
they are recognized properly. The underlying issue
couldn't be identified yet, but since we know what
we get let's just fall back to the old boot device
detection and print an according warning message.

Thanks: to mika and Haudegen for debugging
Closes: #766964