Add Arrays::getFirstIfSet

Use case: I want to select the value of `username` if it is not null or select the value of `email` otherwise generate a unique username for the user.

```php
$post = ['username' => null, 'email' => 'chadicus@example.com', 'password' => 'secret'];
$username = Arrays::getOne($post, ['username', 'email'], generateUsername());
```

Let me know if you have any questions.
mixed is right I believe. But needs a small description.