ASP newb question

I need to ask a quick question. how can I trap a null, if I am going to CStr() it if it has a string in it?

Example:

If (false Or CStr(Session("UserAuthorization"))="") Or (InStr(1,authorizedUsers,Session("UserAuthorization"))>=1) Then

Sometimes it comes back null spitting the error 

Invalid use of Null: 'CStr'

So how would I go about trapping, rather checking for a null and then going about my bizzzzness. all help would be greaty apreciated.