How to optimize query
It's not clear what you want but this may be it:

SELECT
    Name,
    SUM(CASE WHEN Used IS NULL 
           THEN ItemQty 
           ELSE 0 
        END) AS QtyNotUsed
FROM 
    ContainersReceived
GROUP BY
    Name ;


To be efficient, add an index on (Name, Used, ItemQty)



If you have another index, on (Used, Name, ItemQty), this query will be even more efficient - but it will not show Names that have no row with Used IS NULL:

SELECT
    Name,
    SUM(ItemQty) AS QtyNotUsed
FROM 
    ContainersReceived
WHERE
    Used IS NULL 
GROUP BY
    Name ;