The mdbTable does not sort correct when having property values that is NULL.

When debugging, the following (compare) function will return 0 if a value is NULL.

 function (a, b) {
        /** @type {?} */
        var i = 0;
        while (i < key.length) {
            a = a[key[i]];
            b = b[key[i]];
            i++;
        }
        if (a < b) {
            return _this.sorted ? 1 : -1;
        }
        else if (a > b) {
            return _this.sorted ? -1 : 1;
        }
        else {
            return 0;
        }
    }));