merge_pr_50464

Ensure that unknown match results don't get overwritten in selector lists.

Differential Revision: https://phabricator.services.mozilla.com/D235205

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1942695
gecko-commit: bc9e2229b06cc721840598cbf4f7d1908928fa58
gecko-reviewers: firefox-style-system-reviewers, emilio