function erase_empty_date_fields( $user_id, $user_meta ) {

    if( empty( $user_meta['nordic_walking_trainer'] ) ) {
        delete_user_meta( $user_id, 'nordic_walking_trainer' );
    }

    if( empty( $user_meta['beginn_nordic_walking_trainer_online_zugang'] ) ) {
        delete_user_meta( $user_id, 'beginn_nordic_walking_trainer_online_zugang' );
    }

}

add_action( 'wpf_user_updated', 'erase_empty_date_fields', 10, 2 );