[BUGFIX] Avoid notice in AbstractUserAuthentication

Use an empty string instead of null as default value
for username and password to avoid warnings with PHP8.

Resolves: #100739
Releases: main, 12.4, 11.5
Change-Id: Ie06431f27cab919a7df8467f1b5711d7b2b42291
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78816


Tested-by: core-ci <typo3@b13.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>