how?

here's the code to populate a mailchimp field with a wordpress variable:

add_filter( 'mailchimp_sync_subscriber_data', function( MC4WP_MailChimp_Subscriber $subscriber, $user ) {
	$subscriber->merge_fields['MERGE6'] = AFF_ID_GOES_HERE;
	return $subscriber;
}, 10, 2 );


here's the code to get the aff id:
if ( $affiliate = affiliate_wp()->affiliates->get_by( 'user_id', $user->ID ) ) {
    // do stuff
}

how do i combine the two so i can push the affiliate ID to MERG6, only if it's not null (i.e. the user isn't an affiliate?

i'm trying to push the affiliate ID to mailchimp, thus letting me automate based on a changed/added merge field.