[BUGFIX] Avoid possible notice in AbstractPlugin

Checking for a not existing value of a flexform sheet must not lead
to a notice.

Resolves: #98483
Releases: main, 11.5
Change-Id: I8ea7f6e8ecafcd14cc572e4c7207dc5e67300a18
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75896


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>