Qnx: Do not activate any API level if none is selected

The code was introducted before the "default configuration"
was implemented.

If the user deactivates all the BlackBerry Api levels, and restarts
Qt Creator, BlackBerry kits will get generated again, unless all
the API levels are completely uninstalled.

Change-Id: Ie5525d3c08009fb3df986802cd25da24c290098b
Reviewed-by: David Kaspar <dkaspar@blackberry.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>