[BUGFIX] Deal with nullable pages['keywords'] field in IS

The BE may crash when adding a 'keyword' to a page record
using the indexed_search BE module: Field 'keywords' of
table 'pages' may be null, the call to GU::trimExplode()
has to be sanitized for this case since #101453.

Resolves: #103542
Related: #101453
Related: #103541
Releases: main
Change-Id: If8bdabd629f35a131b19de1bdd3ac5057c1d4c61
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83676


Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Garvin Hicking <gh@faktor-e.de>