If the activation fails even before the active connection instance is
created, we get the following:

 $ nmcli connection up vpn1
 libnm-CRITICAL **: nm_active_connection_get_connection: assertion 'NM_IS_ACTIVE_CONNECTION (connection)' failed
 nmcli-CRITICAL **: active_connection_hint: assertion 'connection' failed
 Error: Connection activation failed: Not authorized to control networking.

Check that we have an active connection before showing the hint.

Fixes: bc6c042d