Try using exclude filter instead of using Include in this section. Exclude all the value list except actual Null and Textual Null.
Can you also discuss the case more? It will help all of us to understand what you are trying to achieve and is there any alternative of doing it then what you are following.. Also mention output that you are expecting.
Custom value list is used when you have total 1000 values but you get list of 250 values and someone asks to check you if they are available in the data or not. You can directly copy and paste the values to the custom value list. This avoids the manual selection of 250 values in the filter section which might consume lot of time.
Thanks for your time... Switching the filter type to exclude would work in this case to include the null literals, but it doesn't help if the domain of the field is changing. Using "exclude" to include a specific list of values will include extra values if they are new to the data set.
I work with data source filters a lot (that are dependent on parameters), and using the "select from list" will show members after the data source filter is applied. What if that parameter changes? Then the possible values for the "select from list" changes. This is why I use "custom value list" so I know that I have a very specific list of values included whether they are in the domain or not. Does that make sense?
I use the copy/paste feature all the time. Love it! I just wish I could paste in a null literal