I discovered something some time ago, and could have sworn I wrote a blog article about it, because I thought it was a tip that should be shared with the world. Today, I stumbled across the same problem again, and went to look up my post... yes I refer back to my own posts to refresh my recollection from time to time.
Anyway, I discovered I had neglected to write an article about it, but found the issue mentioned in a broader article written by luminary Dave Musgrave called Understanding how Dynamics GP works with SQL
In this article Dave explains that since Dynamics GP grew up on databases like Btieve and Ctree, and these databases typically avoided the occurrence of NULL values in the data set by initializing number and currency fields with Zero and String fields with blanks. Consequently, Dynamics GP doesn't intuitively know how to deal with NULL values.
This is important for users who are working with SmartList tools like SmartList Builder or Designer.
One of the best inherent features in SmartLists is the ability to search fields based on values or the lack thereof. If a NULL value is returned in a SmartList field, then there is no way to search/restrict your results based on the return of a NULL value.
The easiest way to resolve this issue is during the creation of the view your SmartList is built on. If you have a field returning NULL values, it is best to transform these null values into blanks instead.
Here is an example of how to make this work.
CASE WHEN TABLE.FIELD IS NULL THEN ''
END 'Field Label'
This will result in SQL and ultimately your SmartList column returning either nothing or the actual value in the field. The value nothing can be returned by searching for blanks in the field.
Failure to do this will both clutter your SmartList and limit the its functionality, so I highly recommend taking this into account when building SQL Queries, Views and SmartLists.
NOTE: I did in fact write a blog post about this last October, and apparently mislaid it. Here is a link.
No filters on Null Values in SmartLists