[BUGFIX] Declare setFlatConstants() returns void

Add a missing return value in
ConstantAwareTokenStream->setFlatConstants().

Resolves: #103976
Related: #97816
Releases: main, 12.4
Change-Id: Ia4d10f846db2bf26f721c5eb03781d3592073a08
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84493


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>