Put an empty Rectangle item to the right of your data tablix, with it's height as the minimum height you want the report body to take up. Even if the tablix has no data, the Rectangle will act as a minimum height.
In my screenshots the rectangle is blue and the footer is yellow to highlight what happens. You can make your rectangle whatever colour and width you want. From my testing in SSRS 2008 R2, this still works even if the rectangle has no colour and the report's ConsumeContainerWhitespace
property is True.


If you're printing the report or exporting it to PDF etc., you shouldn't have to worry about this. The page size of the print/export will automatically put the footer at the very bottom of each page, regardless of the body contents.