Have you tried right clicking the "Null" header and clicking "Edit Alias"? You can just put a space or empty text to hide the words "Null". Hope this helps.
Have you tried right clicking the field in the data pane and clicking "Aliases"? You may be able to set the alias for null there.
Since this is a null Date value I'm not sure if there is a way to effectively hide it. One workaround might be to create calculated fields to display the Year, Month, Day using a formula similar to:
IF ISNULL([Enrollment Period]) THEN " "
ELSE STR(DATEPART('month',[Enrollment Period]))
Basically, you end up converting the different parts of the Date to Strings so that you can replace the null values with a blank space.
Hope this helps