Issue #3255637 by Liam Morland, 3li, joegraduate, rivimey, alexpott, mondrake,...

Issue #3255637 by Liam Morland, 3li, joegraduate, rivimey, alexpott, mondrake, ravi.shankar, hswong3i, ameymudras, longwave, Anchal_gupta, neclimdul, Martijn de Wit, sgourebi, pradhumanjainOSL, gambry, kim.pepper: Deprecate NULL values in Html::escape(), ::decodeEntities(), and FormattableMarkup::placeholderFormat() to make it easier to upgrade to PHP 8

(cherry picked from commit 7430cc3c)