Do not use null for dest

Using a null value for dest causes the drush command to look like "drush runserver 8080 null". The dest (or src) property is required since grunt-drush uses `this.files` to parse target configuration in the multitask (https://github.com/nickpack/grunt-drush/blob/master/tasks/drush.js#L25). Also, dest is always added to the arguments due to a [bug](https://github.com/nickpack/grunt-drush/pull/8) in the way grunt-drush checks whether or not it is defined. Using an empty string, dest is still added to the arguments, but doesn't have the potential to cause any unintended side-effects.

Related issues:
- https://github.com/nickpack/grunt-drush/issues/7
- https://github.com/nickpack/grunt-drush/pull/8