fixes an undefined index when getAccessList returns an empty array

- [] is a valid return value that should be honored as having no access

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

apps/workflowengine/lib/Entity/File.php

@@ -140,7 +140,7 @@ public function isLegitimatedForUserId(string $uid): bool {
 				return true;
 			}
 			$acl = $this->shareManager->getAccessList($node, true, true);
-			return array_key_exists($uid, $acl['users']);
+			return isset($acl['users']) && array_key_exists($uid, $acl['users']);
 		} catch (NotFoundException $e) {
 			return false;
 		}