[BUGFIX] Avoid undefined array key "parent." in AbstractPlugin

Resolves: #99085
Releases: main, 11.5
Change-Id: Ie3b337b93f43fb2c41d76427f29d793b57e9232b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76598


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/frontend/Classes/Plugin/AbstractPlugin.php

@@ -411,7 +411,7 @@ class AbstractPlugin
             $conf['no_cache'] = true;
         }
         $conf['parameter'] = $altPageId ?: ($this->pi_tmpPageId ?: 'current');
-        $conf['additionalParams'] = $this->conf['parent.']['addParams'] . HttpUtility::buildQueryString($urlParameters, '&', true) . $this->pi_moreParams;
+        $conf['additionalParams'] = ($this->conf['parent.']['addParams'] ?? '') . HttpUtility::buildQueryString($urlParameters, '&', true) . $this->pi_moreParams;
         return $this->cObj->typoLink((string)$str, $conf);
     }