code: function _guardPromiseAll (promises, all, callback) {
    if (promises.length === 0) {
      callback()
    } else {
      all(promises).then(() => {
        callback()
      })
    }
  }
docs: Simple function to guard Promise.all
  @function _guardPromiseAll
  @param  {Array<Promise>} promises Array of promises
  @param  {Function} all      Promise all function
  @param  {Function} callback Function to call after all promises finished