drivers: modem: fix sending with zero timeout and no semaphore
drivers: modem: fix sending with zero timeout and no semaphore

There was a regression introduced when reordering function parameters
check. It is valid to pass NULL semaphore with zero timeout, so fix
parameter check to respect that.

Fixes: 67976f8686bb ("drivers: modem: verify send semaphore before
  taking any action")
Signed-off-by: Marcin Niestroj <63506c06cfbc47ace147db1702f6e751f5ac2132@grinn-global.com>

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