replace_empty_woocommerce_tabs_description.php

<?php 
//Ham kiem tra, neu san pham khong co mieu ta se thay the bang tab mieu ta custom 
add_filter( 'woocommerce_product_tabs', 'woo_custom_description_tab', 98 );
function woo_custom_description_tab( $tabs ) {
	$id_product = get_the_id(); //get id product
	$temp = get_post($id_product)->post_content; //get content
	if($temp){ //kiem tra
		return $tabs;	
	}else{
		$tabs['description'] = array(
			'title' 	=> __( 'New Product Tab', 'woocommerce' ),
			'priority' 	=> 5,
			'callback' 	=> 'woo_custom_description_tab_content'
		);
		
		return $tabs;
	}
	
}
//Ham noi dung tab 
function woo_custom_description_tab_content() {
echo '<h2>Custom Description</h2>';
echo '<p>Here\'s a custom description</p>';
}

?>