After some recent changes, searching for empty Multilinks (using the filter method of the hyperdb Database class) is possible using an empty list, e.g., in the filterspec we would have:

{nosy: []}

searching for issues with an empty nosy Multilink (there should be none in a normal roundup installation but I'm using this example for lack of a better one). Note that the specified and documented way of searching for empty Link/Multilink properties is by using '-1' or ['-1'] in the filterspec above instead of the empty list.

The same does not seem to work for Link properties. See the failing test in testFilteringLink where we look for an empty assignedto property.

It also looks like the query generator fails for use-cases where a Link property is searched for *and* used for sorting (or probably grouping, too). See mailinglist archive:
https://sourceforge.net/p/roundup/mailman/message/37280964/

So the search for Link properties needs a similar overhaul like the search for Multilink properties got, see also issue2551119