Show EVERY value in a column to see "null" in other columns

I use MySQL and I want to make a table that displays: by product category, the turnover for each month for the last 12 months regardless of when the request was made.

Some months, some products are not sold.

I would like that for EACH MONTH, ALL the categories are displayed (7) and that the empty values in "total_sales" are indicated in NULL. But the months must be displayed even if the total_sales is NULL

Do you have any ideas? I've been looking for several hours but I can't find anything

Here is my code :

SELECT
    p.productLine,
    DATE_FORMAT( o.orderDate, "%M" ) AS month,
    SUM( od.quantityOrdered * od.priceEach ) AS total_sales 

FROM
    products AS p
    LEFT OUTER JOIN orderDetails AS od ON p.productCode = od.productCode
    LEFT OUTER JOIN orders       AS o  ON od.orderNumber = o.orderNUmber

WHERE
    o.orderDate BETWEEN DATE_ADD( NOW(), INTERVAL - 12 MONTH ) AND NOW()

GROUP BY
    MONTH(o.orderDate),
    p.productLine

ORDER BY
    o.orderDate DESC

Here is the actual result: