[BUGFIX] Return empty string in case the folder does not exist

Currently, when linking to a non-existing folder in and
the user clicks on save an error is thrown.

This has been fixed by returning early with a empty string
in case the folder object is null.

Resolves: #102014
Releases: main, 12.4
Change-Id: I548737fae5e6957fe59bf414b70737bea28700b5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81178


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>

typo3/sysext/core/Classes/LinkHandling/FolderLinkHandler.php

@@ -46,6 +46,10 @@ class FolderLinkHandler implements LinkHandlingInterface
      */
     public function asString(array $parameters): string
     {
+        if (!isset($parameters['folder'])) {
+            return '';
+        }
+
         // the magic with prepending slash if it is missing will not work on windows
         return $this->baseUrn . '?storage=' . $parameters['folder']->getStorage()->getUid() .
         '&identifier=' . urlencode('/' . ltrim($parameters['folder']->getIdentifier(), '/'));