PhutilRemarkupHexColorCodeRule: Do not assume that parameter is a string

Summary:
Check type before potentially mangling HTML in a PhutilSafeHTML element.
For now, do not try to apply this renderer when not dealing with a plain string.

Closes T15802

Test Plan: After deleting the corresponding MarkupCache via DELETE FROM phabricator_cache.cache_markupcache WHERE cacheData LIKE "%whatever description on the page%"; check the description of a page, e.g. of /config/edit/security.require-https/ or /config/edit/storage.default-namespace/.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15802

Differential Revision: https://we.phorge.it/D25605