[BUGFIX] Add more safety checks when dealing with buttons in the BE

This patch adds more sanity checks to the SplitButton when rendering
its items. Specifically the existence of getters is checked if those
are not part of the AbstractButton already.

Resolves: #89729
Releases: master, 9.5
Change-Id: I654238e1c9d606596348119374ac283dda1c1713
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62360


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Reviewed-by: Susanne Moog <look@susi.dev>