Add null to param type-hints

[adriansuter]

The last three parameters of the \Slim\Routing\RouteCollector constructor can be null. So this PR would update the docblock type-hints accordingly.

Slim/Slim/Routing/RouteCollector.php

Lines 97 to 104 in 7f36c9e

	public function __construct(
	ResponseFactoryInterface $responseFactory,
	CallableResolverInterface $callableResolver,
	?ContainerInterface $container = null,
	?InvocationStrategyInterface $defaultInvocationStrategy = null,
	?RouteParserInterface $routeParser = null,
	?string $cacheFile = null
	) {

[coveralls]

Coverage remained the same at 100.0% when pulling fc6531e on adriansuter:patch-route-collector-constructor-docblock into 7f36c9e on slimphp:4.x.