[BUGFIX] Fix PHP 8 warning in AbstractXmlSitemapDataProvider

Added a null coalescing operator around the access $item['lastMod'].

Resolves: #100280
Releases: main, 11.5
Change-Id: Idd4de6d647198e10d87ec01d64fa685535ab59aa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78241


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Nikita Hovratov <nikita.h@live.de>