[BUGFIX] Fix undefined array key in XmlSitemapRenderer

When calling the sitemap (type 1533906435) undefined
key warnings are thrown.

This is fixed by properly checking the array key.

Resolves: #94785
Releases: master
Change-Id: I2961a4b776b4e35fc3e489a0cdabd95f4b0602cf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70402


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>