Firebase won't save keys with null values

For the time being I'm using a weird HACK, before storing the object to Firebase I'm converting all the null values to 0. But I want to know much nicer way, please!

function recursivelyReplaceNullToZero(j) {
    for (var i in j){
        if (typeof j[i] === "object") {
            recursivelyReplaceNullToZero(j[i]);
        }
        if (j[i] === null) {
            j[i] = 0;
        }
    }
}
recursivelyReplaceNullToZero(_json);