Allow null as redirect_route

[toooni]

The functionality “Fetching access keys via OAuth2 to be used with an API” mentioned in the README does not necessarily need a redirect_url, thus providing it should not be mandatory. We have an Endpoint (pingen.com) which does not allow redirect_url to be sent in these cases. Also, it wouldn't make much sense if it did because the route must be a valid symfony route, which in some cases must be created without a use.
I also think it makes sense to allow null here because this parameter isn't mandatory in the League\OAuth2\Client\ProviderAbstractProvider (https://github.com/thephpleague/oauth2-client/blob/master/src/Provider/AbstractProvider.php#L454).

[weaverryan]

Thanks Toni - this makes sense :)

[toooni]

@weaverryan Thank you for the fast merge 👍 Just fyi: I did not change anything about the config. redirect_route is still required there, but using redirect_route: null will work. I am not sure if redirect_url should have a default value null since this bundle is mostly used for purposes which require a redirect_route.

[weaverryan]

I am not sure if redirect_url should have a default value null since this bundle is mostly used for purposes which require a redirect_route.

I agree. For the moment, I think we should keep the config required so that it's not "missed".