[BUGFIX] Properly handle array key access in ExtendedTemplateService

This fixes the ExtendedTemplateService->ext_getSetup()
method, which now properly handles array key accesses.

A new early return now also checks for an empty
key and returns the whole setup in this case.
This is a special case, needed for e.g. the
object browser, to still show the full setup
when object lists exist.

Resolves: #94971
Releases: master
Change-Id: I7b6615022ba59dc99929b05744c1cb33805098aa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71461


Tested-by: core-ci <typo3@b13.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>