Don't trigger src="" load sync if there's no current request.
Seems to match what other browsers do. Other browsers seem to fire
events at random some of the time tho, fun stuff as usual.
Differential Revision: https://phabricator.services.mozilla.com/D218603
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1905646
gecko-commit: eacc9aa46ff11b55e34d47e3e32aa6868b12d34d
gecko-reviewers: smaug