Here's one thing you could do. Are you using SQL to fill this dataset? If so, then you can write the SQL statement to handle the cases directly. Use the ISNULL function. So a sample statement would be...
SELECT FieldThatWontBeNULL, ISNULL(FieldThatMightBeNULL,'Null Replacement') FROM MyTable
Then your dataset doesn't need to worry about this. If you need to set this replacement value dynamically, you can just build it into your SQL statement if you are building as a string, or set it up as a parameter if you're going via a stored procedure.
If you still need help, let us know.