[BUGFIX] Cast values passed to htmlspecialchars to string in ArrayBrowser

The $key in ArrayBrowser can sometimes be numeric. Cast the $key to
string before handing it to htmlspecialchars() to prevent type errors.

Resolves: #90883
Releases: master
Change-Id: If5303800a0b8ef25521a8eb028ac0f11f8c5f648
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64005


Tested-by: Georg Tiefenbrunn <georg.tiefenbrunn@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Tiefenbrunn <georg.tiefenbrunn@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>