[BUGFIX] Use correct check and fallback type for plugin itemGroups resolving

In case a plugin, using the `list` CType has
an item group configured, which is not defined
in `list_type>config>itemGroups`, EMU::addPlugin()
tries to get the label from the `CType>config>itemGroups`
definition.

This does now properly work by using the
correct array access in the if clause as
well as the correct fallback value in case
the defined item group does not exists in
`CType>config>itemGroups`.

Resolves: #102841
Related: #102826
Releases: main, 12.4
Change-Id: If50fc533b6b3c107e21d73b7b0b6380c45807093
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82477


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>