ssrs 2008 r2 parameters

In an existing SSRS 2008 R2 report, I need to add some extra parameters to several reports that call subreports. There will be times when the parameters may be empty. The visibility of specific columns and report headers will determine if the column or report header will be displayed. If the parameter(s) are empty, null, do not contain anything, I want those columns and report headerts to not display.

 Can you show me the code that will test to see if a parameter value is null?