How to test if there are no more previous or next image on attachment page?

You need to use the adjacent image link filters, defined in the source of adjacent_image_link() function.

Example:

add_filter( "next_image_link", "cyb_next_image_link", 10, 4 );
function cyb_next_image_link( $output, $attachment_id, $size, $text ) {

    if( $output == '' ) {
        return '<div class="next-image"><span>NEXT</span> <i class="fa fa-angle-right"></i></div>';
    }

    return $output;

}

add_filter( "previous_image_link", "cyb_previous_image_link", 10, 4 );
function cyb_previous_image_link( $output, $attachment_id, $size, $text ) {

    if( $output == '' ) {
        return '<div class="previous-image"><span>PREV</span> <i class="fa fa-angle-right"></i></div>';
    }

    return $output;

}