Making a SUM value 0

I have a query and those items that return null I need to make them a 0 and add the unit to it as well (g, mL) which comes from the DefaultContainerSizeUnit table. Any help is greatly appreciated

Here is the query:

SELECT	i.ItemName,				
	(CAST(SUM(					      CASE  
		WHEN VolumeOrNumber IS NULL THEN 0			WHEN AdjustmentTypeID = 1 THEN VolumeOrNumber
		WHEN AdjustmentTypeID = 2 THEN VolumeOrNumber *-1
	END) AS VARCHAR(10))) + ' ' + u.Abbreviation AS InventoryTotal											
FROM Item i					 						
    LEFT JOIN Inventory v ON i.ItemID = v.ItemID
    LEFT JOIN DefaultContainerSizeUnit u ON i.DefaultContainerSizeUnitID = u.DefaultContainerSizeUnitID
GROUP BY i.ItemName, u.Abbreviation
ORDER BY i.ItemName		

Returns: