[BUGFIX] Fix PHP 8.0 warning in ContentObjectRenderer

As found by ext:fluid TypolinkViewHelperTest with
disabled error suppression, ContentObjectRenderer
does not properly check an array key access in a
helper method of typolink().

Change-Id: I8c4410473f89473a623461123d99bada8ba8ee14
Resolves: #95583
Releases: master
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71557
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php

@@ -4669,7 +4669,7 @@ protected function resolveMixedLinkParameter($linkText, $mixedLinkParameter, &$c
         if ($linkParameterParts['additionalParams'] !== '') {
             $forceParams = $linkParameterParts['additionalParams'];
             // params value
-            $configuration['additionalParams'] .= $forceParams[0] === '&' ? $forceParams : '&' . $forceParams;
+            $configuration['additionalParams'] = ($configuration['additionalParams'] ?? '') . $forceParams[0] === '&' ? $forceParams : '&' . $forceParams;
         }
 
         return [