Yeah. Put in a trigger's codition line:
unit ammo "AT4Launcher" < 1
So now when the ammo that the unit has is less than 1 (so there are no more rockets) the trigger will be activated.
I don't know if you meant the AT Launcher or the RPG.
The AT Launcher's name is, in the game, AT4Launcher.
The RPG's name is RPGLauncher.