The base implementation of `_.mean` and `_.meanBy` without support for
iteratee shorthands.

@private
@param {Array} array The array to iterate over.
@param {Function} iteratee The function invoked per iteration.
@returns {number} Returns the mean.
function baseMean(array, iteratee) {
  var length = array ? array.length : 0;
  return length ? (baseSum(array, iteratee) / length) : NAN;
}