[BUGFIX] Warnings in ArrayUtility with some PHP versions

PHP issue https://bugs.php.net/bug.php?id=50688 triggers a warning
in ArrayUtility::sortArraysByKey() on some supported PHP versions
like debian wheezy 5.4.4. Since this is a language error and the
code itself is correct and covered by unit tests, the warning is
now suppressed using the @ operator.

Change-Id: Ie169774d8659e55542f0c831078c00958de257c9
Resolves: #56889
Relases: 6.2
Reviewed-on: https://review.typo3.org/28359
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters