[BUGFIX] Keep empty attributes in RTEHtmlParser

To allow the use of "empty" attributes, such
as `download` in the CKeditor, the RTEHtmlParser
does now keep such attributes while processing.

Resolves: #103698
Releases: main, 12.4
Change-Id: I331066c0abaa604ba68c19f0013b5a077fe8f5f4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83953


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>