[BUGFIX] Avoid undefined array key in AbstractPlugin pi_isOnlyFields

Resolves: #99107
Releases: main, 11.5
Change-Id: If5e9d1acf81bf2913405e18cc38f2a7ad5ee62cc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76643


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>