[BUGFIX] Resolve falsy ENV-variables in YAML Site Configuration values

Using '%env(SOME_VARIABLE)%' within a site configuration YAML
did not resolve contents in SOME_VARIABLE that were
falsy ('false', '0', 0).

This patch now properly converts those falsy values to actual
values.

Tests are added to verify the new behavior and prevent
future regressions. Drive-by fix on Unit tests which did
not actually remove env vars after testing, putenv('var')
is needed instead of putenv('var='), because the latter
will keep the environment var around (with an empty value).

Resolves: #91710
Releases: main, 12.4
Change-Id: I99d64f080737ada38fd58a63f4e04e5b0d8c9ef3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84442


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>