Removed warnings: setting `changed_attributes` instance variable if it is already initialized.

This is a follow up to #13474 

The warning appears when `Model#becomes` is called. It only happens when the model was not changed before the `becomes` call.

**no warning:**

```ruby
post = Post.first
post.title = "new"
post.becomes(Article)
```

**warning:**

```ruby
post = Post.first
post.becomes(Article)
```

```
/Users/senny/Projects/rails/activerecord/lib/active_record/persistence.rb:184: warning: instance variable @changed_attributes not initialized
```

**solution:**

This PR solves the problem by accessing `@changed_attributes` only if it has been defined previously.