[BUGFIX] Return expected value on GU::writeFileToTypo3tempDir success

When TYPO3 is in composer mode (var is outside public path),
GeneralUtility::writeFileToTypo3tempDir now returns NULL for
successful operation as stated in the docblock.

Resolves: #90063
Releases: master, 10.4
Change-Id: I670503e68275047f511f3821f97b1b56d62bb7f8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68721


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Richard Haeser <richard@richardhaeser.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Riny van Tiggelen <info@online-gamer.nl>
Reviewed-by: Richard Haeser <richard@richardhaeser.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Riny van Tiggelen <info@online-gamer.nl>