[BUGFIX] Handle empty fragments in HtmlWorker and f:transform.html

HtmlWorker (triggered by f:transform.html view helper) issued a PHP
warning concering an empty Document Fragment when the markup to be
parsed is an empty string (`''`).

Resolves: #95867
Releases: master
Change-Id: I9018274d9f16b20550a8ac5b740753e33737b867
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72210


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>