Throw exception if mink has not been set in context

Throw an exception if the context has not set rather than:

````
PHP Fatal error:  Call to a member function getSession() on null in /home/daniel/www/sulu-cmf/sulu-standard/vendor/behat/mink-extension/src/Behat/MinkExtension/Context/RawMinkContext.php on line 102
````

This happens when mink has not been configured in `behat.yml`.
please use ``null === $this->mink`` instead