Allow transitioning from a custom property to None.
If getComputedStyle(...).getPropertyValue is called on a registered
custom property that is used in a transition and that custom property's
registration is removed, to can be None.
Differential Revision: https://phabricator.services.mozilla.com/D211947
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1899446
gecko-commit: ca745191b7c585065a58a7dd73f9f6eefbb8d605
gecko-reviewers: firefox-animation-reviewers, firefox-style-system-reviewers, boris