twister: change serial option existence verification
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 <77503f07a08667f008ecbe02e9a32eb51adc160d@nordicsemi.no>

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