[BUGFIX] Prevent possible PHP crash with empty systemLocale

When $GLOBALS['TYPO3_CONF_VARS']['SYS']['systemLocale'] is empty, a call
to setlocale() may lead to a PHP crash.

Resolves: #102343
Releases: main, 12.4
Change-Id: Icbb9d0118327c0303ae6d3fc98e1cbecfb704877
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81704


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Benni Mack <benni@typo3.org>