[BUGFIX] Make RichTextElement PHP 8 compatible

RTE yaml configuration can be incomplete/wrong or empty.
Do not access configuration settings without a safeguard.

Resolves: #96383
Releases: main, 11.5
Change-Id: Ie0888402e64a669816edd200d2d54e401e0c399f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72702


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/rte_ckeditor/Classes/Form/Element/RichTextElement.php

@@ -161,7 +161,7 @@ class RichTextElement extends AbstractFormElement
 
         $resultArray['html'] = implode(LF, $html);
 
-        $this->rteConfiguration = $config['richtextConfiguration']['editor'];
+        $this->rteConfiguration = $config['richtextConfiguration']['editor'] ?? [];
         $resultArray['requireJsModules'][] = $this->loadCkEditorRequireJsModule($fieldId);
 
         return $resultArray;
@@ -347,7 +347,7 @@ class RichTextElement extends AbstractFormElement
             'customConfig' => '',
         ];
 
-        if (is_array($this->rteConfiguration['config'])) {
+        if (is_array($this->rteConfiguration['config'] ?? null)) {
             $configuration = array_replace_recursive($configuration, $this->rteConfiguration['config']);
         }