The bees should make it for a day or two without liquid feed. Optimally, you would have checked to see if the syrup was being immediately used, or used currently and stored as well. Presumably these are new packages installed on foundation? What I would have been looking for is if some of the cells being drawn by the bees are being used for storage. If it were me, and I'm writing from Maine where I haven't see the inside of a bee hive since last November, I'd refill the feeder. If they don't take the syrup for a day or so until things warm up no big deal, It doesn't sound like it will be cold enough long enough for the syrup to freeze. Just long enough for the bees to form a cluster and perhaps stop taking syrup for a day or so.