To identify products with no sales in MDX we can use the isBlank function and a filter.

Here is a short sample

SELECT [Measures].[Internet Sales Amount] ON 0,
FILTER(
	[Product].[Product].[Product], 
	ISEMPTY([Measures].[Internet Sales Amount])
) ON 1
FROM [Adventure Works]

And if you want to have products with sales you can use the NON EMPTY function.

SELECT [Measures].[Internet Sales Amount] ON 0,
NON EMPTY [Product].[Product].[Product] ON 1
FROM [Adventure Works]