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: 67976f86 ("drivers: modem: verify send semaphore before
  taking any action")
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>