Let set() replace null with a node

Sometimes, a block is optional (eg: the `finally` block in a try statement). Currently, the `set` method doesn't know how to handle each case appropriately.

```js
// This *should* work, but it doesn't.
node.set('finalizer', 'console.log("test")')

// In this case, there's a workaround.
node.after(`finally {
  console.log("test")
}`)
```