twister: change serial option existence verification

Modify serial and serial_pty option availability verification in DTU
object in device_is_available method to avoid situation when empty string
passed as serial in hardware_map.yaml cause hang up Twister in this
place.

Fixes: #41169

Signed-off-by: Piotr Golyzniak <piotr.golyzniak@nordicsemi.no>
1 file changed