[BUGFIX] Harden type annotations around user authentication handling

When authenticating and fetching user records, appropriate methods
normally pass around either the full database record or `false` in case
the user does not exist or could not be authenticated. The expected
types are now added to all relevant methods in authentication services.

Resolves: #99544
Releases: main, 11.5
Change-Id: Ib45440a39e64b5ac5a1f1c4799e2f59cb13526bf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77335


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>