core: allow gpiod_line_set_value_bulk() to accept null values

core: allow gpiod_line_set_value_bulk() to accept null values

Change gpiod_line_set_value_bulk to interpret a NULL values pointer as an
array of zero, as per gpiod_line_request_bulk, gpiod_line_set_config_bulk,
and gpiod_line_set_direction_bulk_output.

The old behaviour was to segfault.

Add a corresponding test case for gpiod_line_set_value_bulk().

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>