Fatal error when using Nothing::flatMap

Consider the following code:

```php
$maybe = Maybe::fromValue(null)
    ->flatMap(function () {
        return Maybe::fromValue('some value');
    })
    ->getOrElse('default');
```

It causes the error: `PHP Fatal error:  Uncaught Error: Call to a member function getOrElse() on null`. Probably `Nothing::flatMap` should return an instance of `Nothing` instead of `null`.