Return types are too strict

[cicalese]

#327 introduced several return types that are too strict. Some examples,

Sometimes an identity provider will return an opaque access token. When an attempt is made to json_decode the JWT, null is returned. This gives the following exception when an attempt is made to retrieve the access token payload (seen when authenticating with Google):

TypeError: Return value of Jumbojett\OpenIDConnectClient::decodeJWT() must be an instance of stdClass, null returned
Backtrace:
from /var/www/mediawiki/w/extensions/OpenIDConnect/vendor/jumbojett/openid-connect-php/src/OpenIDConnectClient.php(1231)
#0 /var/www/mediawiki/w/extensions/OpenIDConnect/vendor/jumbojett/openid-connect-php/src/OpenIDConnectClient.php(1751): Jumbojett\OpenIDConnectClient->decodeJWT()
#1 /var/www/mediawiki/w/extensions/OpenIDConnect/includes/OpenIDConnect.php(306): Jumbojett\OpenIDConnectClient->getAccessTokenPayload()

Sometimes an identity provider will not return a refresh token. In this case, $this->refreshToken is never changed from the default value of null. This gives the following exception when an attempt is made to retrieve the refresh token (seen when authenticating with Microsoft):

TypeError: Return value of Jumbojett\OpenIDConnectClient::getRefreshToken() must be of the type string, null returned
Backtrace:
from /var/www/mediawiki/w/extensions/OpenIDConnect/vendor/jumbojett/openid-connect-php/src/OpenIDConnectClient.php(1732)
#0 /var/www/mediawiki/w/extensions/OpenIDConnect/includes/OpenIDConnect.php(323): Jumbojett\OpenIDConnectClient->getRefreshToken()