Welcome to HBT!
The airlock will keep the nasties out even without the liquid so don't worry (though do try and remember in the future, it's an extra layer of security). Also bubbling is not a good indication of fermentation, any leaks in the seal of the bucket or the airlock grommet or bung will let the CO2 escape.
If there is a layer of foam (called krausen) on the beer then it is definitely fermenting properly. Once the krausen has fallen you can check the gravity and see how it's doing. Once it has been stable for 3 days you can bottle the beer.