[12.x] Allow CachedState properties to be nullable

[jackbayliss]

I'm using WithCachedRoutes in my test case - I have a test where I check various permissions, the routes in my web.php are flagged, depending on the feature so I need to make sure the test is refreshed as older cached routes don't have the permissions I'm checking.

I'd like the ability to do the below in my test - without the need for a new test case etc:

    public function setUp(): void
    {
        // We ensure each test has freshly cached routes due to testing feature based permissions below.
        CachedState::$cachedRoutes = null;

        parent::setUp();
    }

I've adjusted both properties to allow for null to keep them inline. There could probably be further adjustments to allow for methods to do this like withoutCachedRoutes for better dx, but for now I just want to do it myself.

Dont think it's a breaking change, but open to feedback - cheers 👍🏻

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[jackbayliss]

Failing tests look to be cURL error 28: SSL connection timeout issues, so not related.