Make _.merge align with _.assign exit early snippet.

lodash.js

@@ -6760,13 +6760,13 @@
      * // => { 'fruits': ['apple', 'banana'], 'vegetables': ['beet', 'carrot] }
      */
     function merge(object, source, guard) {
-      if (!object) {
-        return object;
-      }
       var args = arguments,
           length = args.length,
           type = typeof guard;
 
+      if (!object || length < 2) {
+        return object;
+      }
       // enables use as a callback for functions like `_.reduce`
       if ((type == 'number' || type == 'string') && args[3] && args[3][guard] === source) {
         length = 2;