[BUGFIX] Prevent PHP 8.1 runtime deprecation notice in IconRegistry

IconRegistry->detectIconProvider() may receive a null value for
the $iconReference parameter. E.g. in
IconRegistry->registerModuleIcons(). Therefore, a cast to string is
needed for strtolower.

Resolves: #99152
Releases: main, 11.5
Change-Id: Ia45c8a52ff12fa4d988977e1265b204df8217d2b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76756


Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>