Filtered results 404 beyond the end of the list

@fg got to https://debusine.debian.net/debian/r-fabiang.dhcargo/workflow/?filter-workflow_templates=publish-to-sid-dh-cargo&page=3&filter-statuses=running__completed

by clicking through filter additions after moving off the first page. The result is a (bare) 404 page, rather than a "no results found" that you'd see on the first page of a filter