You're 8.7 unpatched? if that's actually the case, upgrade! You're missing out on the extra operators for queries that will make this less of a headache.
Also, what is the end result of this report? If you are getting Null listings, it's because there's no value in that spot in the inventory return for that query.
To test, I designed a report called 'Power Users Report'. The query filter is "Computer"."Local Users and Groups"."Local Groups"."Name" = "Power Users" and then I have Column Set of Device Name and also Local Users and Groups.Local Users.Member with a qualifier of "Computer"."Local Users and Groups"."Local Groups"."Name" = "Power Users". When run, this shows me all devices with entries under 'Power Users' and what the names are in that group. if it shows 'Null' it's because there's nothing in that group on that machine. Does this help at all?
Sp 5 is on my "To Do" list. I've just been busy with other things...
I was asking for <> (null), because I only want to see those computers with one or more Power User member(s). If there are no members, which is (null), then I want to skip them. It works in SQL, but not here. Maybe patching LD will fix it.
Upgrading the core will give you the 'Like' and 'Not Like' operators - in which case you should be able to add a 'Not Like = (null) to the report to have it exclude those machines.