bindings: python: change the return value of gpiod_LineBulk_event_wait()

bindings: python: change the return value of gpiod_LineBulk_event_wait()

gpiod_Line_event_wait() works differently than its LineBulk
counterpart. While the latter returns either a list of lines or False,
the former returns True if an event occurred on this line or False
otherwise.

False is not a good counterpart for an actual object, so instead return
None if no events occurred on any of the monitored lines.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>