[BUGFIX] Avoid invalid type exception in FinisherVariableProvider

Guard invalid argument type exception by using null coalescing operator
in `\TYPO3\CMS\Form\Domain\Finishers\FinisherVariableProvider`.

Resolves: #97699
Releases: main, 11.5
Change-Id: I6312fb35d52857004e0467a20e215fc4095e0037
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74832


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/form/Classes/Domain/Finishers/FinisherVariableProvider.php

@@ -97,7 +97,7 @@ final class FinisherVariableProvider implements \ArrayAccess, \IteratorAggregate
     public function exists($finisherIdentifier, $key): bool
     {
         try {
-            ArrayUtility::getValueByPath($this->objects[$finisherIdentifier], $key, '.');
+            ArrayUtility::getValueByPath($this->objects[$finisherIdentifier] ?? [], $key, '.');
         } catch (MissingArrayPathException $e) {
             return false;
         }