fix GetPreferenceViewHelper error on missing key

ot_templating/Classes/ViewHelpers/Template/GetPreferenceViewHelper.php

@@ -13,7 +13,7 @@ use TYPO3\CMS\Extbase\Object\ObjectManager;
 use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
 
 /**
- * Returns the value of the requested preference
+ * Returns the value of the requested preference, or null if the key does not exist.
  *
  *     {namespace ot=Opentalent\OtTemplating\ViewHelpers}
  *
@@ -32,7 +32,7 @@ class GetPreferenceViewHelper extends OtAbstractViewHelper
      * -- This method is expected by Fluid --
      * Declares the viewhelper's parameters
      */
-    public function initializeArguments()
+    public function initializeArguments(): void
     {
         $this->registerArgument(
             'key',
@@ -46,16 +46,15 @@ class GetPreferenceViewHelper extends OtAbstractViewHelper
      * -- This method is expected by Fluid --
      * Renders the content as html
      *
-     * @return string
+     * @return string | null
      * @throws NoSuchWebsiteException
      */
-    public function render(): string
+    public function render(): ?string
     {
         $website = $this->otWebsiteRepository->getCurrentWebsiteFromFEGlobals();
         $preferences = $this->templateRepository->getTemplatePreferences($website);
 
-        $key = $this->arguments['key'];
+        $key = $this->arguments['key'] ?? null;
         return $preferences[$key];
     }
-
 }