After some experimentation I have found that on an actor array I can use “Set Array Element” and connect the array pin and the index pin adn if you leave the item pin unconnected it will set the array element to “None”. You can then do a “Find Item” on the Array and if you leave the Item pin unconnected it will return the first index it finds that is empty or “None” and if all indexes are set it will return -1.