Filtering for "None" on "Single Version Picker" field doesn't work.

Selecting "Unknown" for a version picker field in the filter screen has no effect on the list issues returned.
In other words, selecting "Unknown" will not limit the list of returned issues. Issues that have no value in this field and issues that have any value in this field will be returned which makes it pointless.

I think the search should allow the user to select "Any" or "No Version" just as the "Affects Versions" field allows.