In Twilight Imperium, when you activate a system, you don't have to move units. The rules for Movement say:
The player may choose to not move any ships.
This rule appears to be for the purpose of letting you use production abilities in a system without needing to move units there. But can you still activate a sector, moving no units, if you don't have any units there?
This would let you do two things. First, you could spend a tactics token to delay acting (similar to the race ability of The Yssaril Tribes). Second, if done on a system adjacent to one or more of your PDS II's, this would let you fire a volley of space cannons at the ships without committing any units.