gh-96290: Support partial/invalid UNC drives in ntpath.normpath() and splitdrive() (GH-100351)

This brings the Python implementation of `ntpath.normpath()` in line with the C implementation added in 99fcf15

Co-authored-by: Barney Gale <barney.gale@gmail.com>
Co-authored-by: Eryk Sun <eryksun@gmail.com>