The simplest way I find (both with Pivot & Straight Tables) is to add the expression ='' and give it a label of ='' - simple. Also use the background colour format to set darket if required.
The other thing this is useful for is returning data for all dimensional values; say we have 100 stock items but sales have only been recorded against 80. Using a table with Stock as dimension and =sum(sales) as the expresion we would only return the 80 items that had sales. By adding ='' as a second expression we return values for all dimension values - ie showing those stock items that haven't had any sales as well which often is just as important as what has been sold.
Hope that helps.
Matt - Visual Analytics Ltd