fix: set displayname to email if no other is specified

this ensures that hashed uid guests have a sensible display name

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

lib/GuestManager.php

@@ -77,6 +77,8 @@ public function createGuest(?IUser $createdBy, string $userId, string $email, st
 
 		if ($displayName !== '') {
 			$user->setDisplayName($displayName);
+		} else {
+			$user->setDisplayName($email);
 		}
 
 		if ($language !== '') {