Support for result => undefined

[deltazero-cz]

Hi, I've made a fork which adds support for desired result being undefined.

Real life usage:

/**
 * Returns the lowest of array's values.
 * If empty, returns undefined.
 * If not all values are numeric, returns NaN.
 *
 * @function
 * @returns {number|undefined}
 *
 * @example <caption>returns 1</caption>
 *    [1, 2, 3, 4, 5, 6].min()
 *    // => 1
 *
 * @example <caption>returns 1</caption>
 *    [{num: 1}, {num: 2}, {num: 3}, {num: 4}, {num: 5}, {num: 6}].map(r => r.num).min()
 *    // => 1
 *
 * @example <caption>returns NaN</caption>
 *    [1, 2, 3, 'Four', 5, 6].min()
 *    // => NaN
 *
 * @example <caption>returns undefined</caption>
 *    [].min()
 *    // => undefined
 */
Array.prototype.min = function() {
  return this.length
      ? Math.min(...this)
      : undefined
}

[coveralls]

Coverage increased (+0.2%) to 77.202% when pulling c3edabb57de66585df10c593021dcfccb818cf2f on deltazero-cz:master into ccd6cdbff8a4db98d20da8ac1868558a45913005 on yamadapc:master.

[yamadapc]

Thank you