[TASK] Simplify code in ConjunctionValidator

The code in ConjunctionValidator can be greatly
simplified by taking advantage of the fact that
an empty result object can be merged with a future
one. That eliminates an unnecessary code path as
well as phpstan warnings.

Used command:

> Build/Scripts/runTests.sh -s phpstanGenerateBaseline

Resolves: #97623
Releases: main, 11.5
Change-Id: I6f10c25562044adb5cbe283255ea07e71effa9cb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74740


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>