part: Add check for NULL dev_part_str

Some callers (e.g. cmd/fs.c) of fs_set_blk_dev may use a NULL dev_part_str.
While blk_get_device_part_str handles this fine,
part_get_info_by_dev_and_name does not. This fixes commands crashing when
implicitly using bootdevice.

The unit test has also been updated to set bootdevice to a known value and
to restore it after we are done.

Fixes: 7194527b ("cmd: fs: Use part_get_info_by_dev_and_name_or_num to parse partitions")
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>