fix: scaffold over <null> targets in set

This ensures `null` targets are treated as if the value
does not exist during scaffolding.

```javascript
set({ a: null }, [ 'a', 'b', 'c' ], 'C')
```

lib/object.js

@@ -3,7 +3,8 @@ import { forEach } from './collection';
 import {
   isObject,
   isUndefined,
-  isDefined
+  isDefined,
+  isNil
 } from './lang';
 
 
@@ -41,7 +42,7 @@ export function set(target, path, value) {
     var nextKey = path[idx + 1];
     var nextTarget = currentTarget[key];
 
-    if (isDefined(nextKey) && !nextTarget) {
+    if (isDefined(nextKey) && isNil(nextTarget)) {
       nextTarget = currentTarget[key] = isNaN(+nextKey) ? {} : [];
     }
 

test/object.spec.js

@@ -400,6 +400,16 @@ describe('object', function() {
           }
         }
       });
+
+      expect(
+        set({ a: null }, [ 'a', 'b', 'c' ], 'C')
+      ).to.eql({
+        a: {
+          b: {
+            c: 'C'
+          }
+        }
+      });
     });