bindings: python: fix gpiod_Chip_find_lines() for nonexistent lines

bindings: python: fix gpiod_Chip_find_lines() for nonexistent lines

Currently if we call gpiod_Chip_find_lines() with some names that
cannot be looked up, we end up getting an exception from the LineBulk
object's constructor (because None is not a Line and LineBulk can only
hold Line objects).

Fix the behavior of gpiod_Chip_find_lines() for nonexistent lines by
raising an explicit exception if the underlying gpiod_Chip_find_line()
call returns None. Also: add a relevant test case.

Fixes: 96c524c4951c ("bindings: implement python bindings")
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>