If your fermentation took off quickly then you should have a flow of CO2 out of the air lock making it more difficult for infectious critters to get in. I would be more concerned if it was in secondary and you didn't put water/sanitizer in the airlock.
However if it took several days for your fermentation to take off then that is a rip breeding ground for infectious critters to come in and play.
With most lemonade batches, the pH is low enough that spoilage organism are unlikely no matter what. You can fement meads with no problem in open fermenters so I wouldn't worry. Just fill the airlock going forward to prevent oxidation as the CO2 production winds down.