Bump to v4.13.0.

_arrayAggregator.js

@@ -2,7 +2,7 @@
  * A specialized version of `baseAggregator` for arrays.
  *
  * @private
- * @param {Array} array The array to iterate over.
+ * @param {Array} [array] The array to iterate over.
  * @param {Function} setter The function to set `accumulator` values.
  * @param {Function} iteratee The iteratee to transform keys.
  * @param {Object} accumulator The initial aggregated object.
@@ -10,7 +10,7 @@
  */
 function arrayAggregator(array, setter, iteratee, accumulator) {
   var index = -1,
-      length = array.length;
+      length = array ? array.length : 0;
 
   while (++index < length) {
     var value = array[index];