I don't know what "send the stuff in the empty teleporter" even means. An empty teleporter is... empty. It has no stuff to send.
Anyway, I don't know what makes no sense to you. The teleporter on Gleba has "Target State" set to "Empty." That means it will not activate unless its target is empty. The target of the Gleba teleporter is the teleporter on Nauvis. The Navis teleporter contains artillery shells; therefore, it is not empty. So, the teleporters will not activate because the target of the "Target State: Empty" teleporter is not empty.