Fix optional callbacks being actually optional (fixes #1223)

lib/internal/filter.js

@@ -1,7 +1,10 @@
 import arrayMap from 'lodash/_arrayMap';
 import property from 'lodash/_baseProperty';
+import noop from 'lodash/noop';
+import once from './once';
 
 export default function _filter(eachfn, arr, iteratee, callback) {
+    callback = once(callback || noop);
     var results = [];
     eachfn(arr, function (x, index, callback) {
         iteratee(x, function (err, v) {

lib/mapValuesLimit.js

@@ -1,5 +1,8 @@
 import eachOfLimit from './eachOfLimit';
 
+import noop from 'lodash/noop';
+import once from './internal/once';
+
 /**
  * The same as [`mapValues`]{@link module:Collections.mapValues} but runs a maximum of `limit` async operations at a
  * time.
@@ -21,6 +24,7 @@ import eachOfLimit from './eachOfLimit';
  * transformed values from the `obj`. Invoked with (err, result).
  */
 export default function mapValuesLimit(obj, limit, iteratee, callback) {
+    callback = once(callback || noop);
     var newObj = {};
     eachOfLimit(obj, limit, function(val, key, next) {
         iteratee(val, key, function (err, result) {

lib/reduce.js

@@ -1,4 +1,6 @@
 import eachOfSeries from './eachOfSeries';
+import noop from 'lodash/noop';
+import once from './internal/once';
 
 /**
  * Reduces `coll` into a single value using an async `iteratee` to return each
@@ -41,6 +43,7 @@ import eachOfSeries from './eachOfSeries';
  * });
  */
 export default function reduce(coll, memo, iteratee, callback) {
+    callback = once(callback || noop);
     eachOfSeries(coll, function(x, i, callback) {
         iteratee(memo, x, function(err, v) {
             memo = v;

lib/sortBy.js

@@ -17,7 +17,7 @@ import map from './map';
  * The iteratee is passed a `callback(err, sortValue)` which must be called once
  * it has completed with an error (which can be `null`) and a value to use as
  * the sort criteria. Invoked with (item, callback).
- * @param {Function} [callback] - A callback which is called after all the
+ * @param {Function} callback - A callback which is called after all the
  * `iteratee` functions have finished, or an error occurs. Results is the items
  * from the original `coll` sorted by the values returned by the `iteratee`
  * calls. Invoked with (err, results).

lib/transform.js

@@ -1,6 +1,8 @@
 import isArray from 'lodash/isArray';
+import noop from 'lodash/noop';
 
 import eachOf from './eachOf';
+import once from './internal/once';
 
 /**
  * A relative of `reduce`.  Takes an Object or Array, and iterates over each
@@ -53,6 +55,7 @@ export default function transform (coll, accumulator, iteratee, callback) {
         iteratee = accumulator;
         accumulator = isArray(coll) ? [] : {};
     }
+    callback = once(callback || noop);
 
     eachOf(coll, function(v, k, cb) {
         iteratee(accumulator, v, k, cb);