Krileon, while we appreciate your help many times, some days you just don't seem to get the issues and are not providing constructive replies.
Here I'm referring to a user doing a search. If he does not choose any value for all the fields criteria and click search, the entire list of members will be shown. Is this your intended feature? Then it is strange. If any site owner wants to allow searchers to see all the members at one go, they can use the "List All" which is available. But your current feature does not allow control on to prevent this. It's a loophole.
Adding filter to the users list is another purpose altogether. So this has nothing to do with whether the members (not searchers) fill in the fields or not.
Possible solutions:
1) When no values are selected, the result is a notification to enter values. OR
2) Let the search function share the same "Required" feature as the one during registration or edit. That forces the searcher to key in a value for specific fields which are "required".
If you have other easier ways, please share.