I forgot to put liquid in my airlock!

It's almost certainly fine. As Ken points out, CO2 production pushes air out of the airlock, particularly at the beginning. The headspace in your fermentation vessel is predominantly CO2, so oxidation isn't much risk at all.

Regarding contamination (a more likely issue anyway), the physical presence of an empty airlock should do a passable job of protecting your brew. Airborne bacteria generally falls "down", so it should have some difficulty with a 3-piece airlock (particularly if there's a gentle breeze of CO2 wafting it back out).

Bugs might still crawl in, depending on where your fermentor is.

So, add some liquid and see how it turns out at the end - you're probably fine.

PS - google "open fermentation" sometime, it will put your mind at ease.

I've had it run dry (push out all the liquid) several times. I also use a cheap vodka, as it doesn't evaporate as fast. I've also had where the fermentation pushes into the airlock, then as activity stops, sucks it back in. Batch still turned out.

at the beginning, co2 production should push air out through the airlock. So, it probably depends on how long it was left that way