[TASK] Fix the return type annotation of getIndpEnv

GeneralUtility::getIndpEnv can also return values other than string,
e.g., PATH_INFO might return null, and TYPO3_SSL will return a
bool. Also, _ARRAY will return an array of all possible scalar
values.

The return type annotation needs to correctly reflect that in order
for static type analysis to be able to work correctly when this
function is called.

Resolves: #97020
Releases: main, 11.5
Change-Id: Ibb24f1a3451c6598452402fc7a7b0a96c66bbd01
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73734


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>