[1.x] Gracefully handles unexpected null scope

[timacdonald]

This PR gracefully handles null scope when a feature definition / resolver has a non-nullable type declaration.

Feature::define('api-beta', fn (User $user) => true);

Auth::logout();

Feature::active('api-beta');

Instead of throwing a type exception when checking if the feature is active, we will now return false and dispatch the Laravel\Pennant\Events\UnexpectedNullScopeEncountered event.

This is a sensible default and is functionality already present in Laravel Authorisation Guards.

If an app wants to opt-out of this behaviour:

use Laravel\Pennant\Events\UnexpectedNullScopeEncountered;

Event::listen(UnexpectedNullScopeEncountered::class, fn () => abort(500));

[timacdonald]

This change means that the feature class stays in-memory once it is defined and isn't re-resolved each time it is checked (it was already being resolved here, however we didn't hold onto it, so it was garbage collected).

Depending on the context, this may actually be a performance gain, as the class isn't being re-resolved everytime the feature is checked - however this would probably have no real-world impact.

It is only really a concern if the user is mutating the class each time it is invoked - but there is an event for doing that kind of thing outside of the feature class.