R1:6ab2b56a1a4a

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…