To do a test for NULL you can use the IsNull function. So your code would look something like this:
If IsNull({?DateType}) then "N/A"
else ...
the problem with this is that a function you can only return one data type, and if your Trx_Date and Settle_Date are type Date (or DateTime) then you can’t return N/A, which is a string.
To get around this you could try returning all values as strings, if you’re just displaying the dates and not sorting or grouping this could work. HTH.