function baseMean(array, iteratee) {
    var length = array == null ? 0 : array.length;
    return length ? (baseSum(array, iteratee) / length) : NAN;
  }
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.