[BUGFIX] Avoid 'Undefined array key "uid"'

In case a new group is imported, the group uid is not set.
Setting it to `0` in this case prevents the issue.

Classes/Controller/ModuleController.php

@@ -552,7 +552,7 @@ public function ajaxGroupsImport(ServerRequestInterface $request): ResponseInter
 			// Merge LDAP and TYPO3 information
 			$group = Authentication::merge($ldapGroup, $typo3Groups[0], $config['groups']['mapping']);
 
-			if ((int)$group['uid'] === 0) {
+			if ((int)($group['uid'] ?? 0) === 0) {
 				$group = Typo3GroupRepository::add($table, $group);
 			} else {
 				// Restore group that may have been previously deleted