static isEmptyObj(obj) {
        for (const prop in obj) {
            if (Object.prototype.hasOwnProperty.call(obj, prop)) {
                return false;
            }
        }
        return true;
    }
Return `true` if the given object is empty
cf. http://stackoverflow.com/questions/679915/how-do-i-test-for-an-empty-javascript-object and http://jsperf.com/empty-object-test

@param {object} obj
@returns {boolean}