Set group usercount to 0, not '', fixes #9782

Fixes an issue with the user count when creating a group, and then a new user.

@MTRichards @blizzz 

To test:
 - Go to settings > users
 - Add a new group
 - Without refreshing page, add a new user with that new group

Previously:
 - User count for the new group would show 'NaN'

Now:
 - User count for the new group should show '1'

See: https://github.com/owncloud/core/issues/9782#issuecomment-50892350