Avoid returning assignDefaults if no object is provided to _.defaults.

lodash.js

@@ -5440,11 +5440,11 @@
      * // => { 'name': 'barney', 'employer': 'slate' }
      */
     function assign(object, source, guard) {
-      if (!object) {
+      var args = arguments;
+      if (!object || args.length < 2) {
         return object;
       }
-      var args = arguments,
-          argsIndex = 0,
+      var argsIndex = 0,
           argsLength = args.length,
           type = typeof guard;
 
@@ -5636,7 +5636,10 @@
      * _.defaults({ 'name': 'barney' }, { 'name': 'fred', 'employer': 'slate' });
      * // => { 'name': 'barney', 'employer': 'slate' }
      */
-    function defaults() {
+    function defaults(object) {
+      if (!object || arguments.length < 2) {
+        return object;
+      }
       var args = slice(arguments);
       args.push(assignDefaults);
       return assign.apply(null, args);

test/test.js

@@ -2983,6 +2983,10 @@
   _.forEach(['assign', 'defaults', 'merge'], function(methodName) {
     var func = _[methodName];
 
+    test('`_.' + methodName + '` should return `undefined` when no destination object is provided', 1, function() {
+      strictEqual(func(), undefined);
+    });
+
     test('`_.' + methodName + '` should return the existing wrapper when chaining', 1, function() {
       if (!isNpm) {
         var wrapper = _({ 'a': 1 });