scripts: gen_handles.py: initialize handle.device
scripts: gen_handles.py: initialize handle.device

Initialize `handle.device` to `None` so that the assert after the
matching has a chance to catch errors. Without this, a failed match will
raise an exception on the line above as it attempts to get a property
that doesn't exist.

Signed-off-by: Jordan Yates <d74a90e280bbad1cc3646781a4e234b467b60fe1@data61.csiro.au>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr