Fix docblock for Model::getEventDispatcher()

[inmula]

The static::$dispatcher property may be null after using Model::unsetEventDispatcher(). The docblock for Model::getEventDispatcher and the docblock for the property itself should reflect that.

This fix avoids warnings from static analysers such as PHPstan when using the getEventDispatcher method:

Using nullsafe method call on non-nullable type Illuminate\Contracts\Events\Dispatcher. Use -> instead.

[driesvints]

We use |null everywhere instead.