Water moves through the trap because gravity is pushing it, not because a vacuum in your lines is pulling it through. As hj pointed out, once past the trap, the water only occupies the bottom half of the pipe and the air in the upper half of the trap arm is more than adequate to prevent "aspiration" before reaching the vent pipe. Could something get blocked and suck the other trap dry? Yes. Could a perfectly placed gust of wind down your roof vent cause the same thing? Yes. Will it? Very unlikely. You are more likely to have a trap evaporate itself dry than to have it aspirated dry.