To adhere to what users are used to to configure the mink-extension drivers.
Let's look how you would enable goutte and sahi using the default configuration values and sahi being the default driver:

    default:
      extension:
        Behat\MinkExtension\Extension:
          default_session: sahi
          goutte: ~
          sahi: ~

Now the configuration for adding wunit, where wunit shall be the default driver:

    default:
      extension:
        Behat\MinkExtension\Extension:
          default_session: wunit
          goutte: ~
          sahi: ~
    
        Behat\YiiExtension\Extension:
          mink_driver: true
          wunit: ~

For this to work (and thus `if (isset($config['wunit'])) {` on the line 61), we need this empty array node.
Please let me know if there is a simpler way to get this to work.
However, enabling wunit by default is **NOT** an option, as one is not required to use wunit when he is using the yii-extension.
He could use as well any of the other mink drivers, e.g. goutte.