[BUGFIX] Add empty alt attribute to decorative images

Decorative images should have an empty alt attribute to hide
them from screenreaders.
See https://www.w3.org/WAI/WCAG21/Techniques/failures/F38

This patch adds the attribute to the images generated by the
icon provider classes using <img> tags.

Resolves: #89653
Releases: master
Change-Id: Ie3186902db3d479b6f2e464fbb578425ced43898
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62304


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>