fix error when using CORS with no auth credentials

[korelstar]

When using an ApiController with CORS annotation, basic authentication is required. However, currently the following exception occurs when a request does not contain any authentication:

Argument 1 passed to OCP\User\Events\BeforeUserLoggedInEvent::__construct() must be of the type string, null given, called in /home/owncloud/nextcloud/lib/private/Server.php on line 577

0.  /home/owncloud/nextcloud/lib/private/Server.php - line 577:
    OCP\User\Events\BeforeUserLoggedInEvent->__construct("*** sensiti ... *", "*** sensiti ... *")

1.  <<closure>>
    OC\Server->OC\{closure}("*** sensiti ... *")

2.  /home/owncloud/nextcloud/lib/private/Hooks/EmitterTrait.php - line 107:
    call_user_func_array(Closure {}, [ "*** sensi ... "])

3.  /home/owncloud/nextcloud/lib/private/Hooks/PublicEmitter.php - line 41:
    OC\Hooks\BasicEmitter->emit("\\OC\\User", "preLogin", [ "*** sensi ... "])

4.  /home/owncloud/nextcloud/lib/private/User/Session.php - line 444:
    OC\Hooks\PublicEmitter->emit("\\OC\\User", "preLogin", [ "*** sensi ... "])

5.  /home/owncloud/nextcloud/lib/private/AppFramework/Middleware/Security/CORSMiddleware.php - line 93:
    OC\User\Session->logClientIn("*** sensiti ... *")

6.  /home/owncloud/nextcloud/lib/private/AppFramework/Middleware/MiddlewareDispatcher.php - line 98:
    OC\AppFramework\Middleware\Security\CORSMiddleware->beforeController(OCA\Notes\Co ... {}, "index")

7.  /home/owncloud/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 119:
    OC\AppFramework\Middleware\MiddlewareDispatcher->beforeController(OCA\Notes\Co ... {}, "index")

8.  /home/owncloud/nextcloud/lib/private/AppFramework/App.php - line 157:
    OC\AppFramework\Http\Dispatcher->dispatch(OCA\Notes\Co ... {}, "index")

9.  /home/owncloud/nextcloud/lib/private/Route/Router.php - line 302:
    OC\AppFramework\App::main("OCA\\Notes\ ... r", "index", OC\AppFramew ... {}, { apiVersion ... "})

10. /home/owncloud/nextcloud/lib/base.php - line 993:
    OC\Route\Router->match("/apps/notes/api/v1/notes")

11. /home/owncloud/nextcloud/index.php - line 37:
    OC::handleRequest()

In consultation with @nickvergessen , this PR fixes this by checking if $user and $pass are not null before trying to login.

Furthermore, I fixed an Undefined index error for PHP_AUTH_USER and PHP_AUTH_PW.

I suggest backporting this to the stable branches.

[korelstar]

ping for review @rullzer @ChristophWurst @LukasReschke @kesselb

[korelstar]

I suggest to Backport this to stable20 and stable21. Can we do this?

[nickvergessen]

/backport to stable21

[nickvergessen]

/backport to stable20