chore: Apply rector strict config again

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

lib/private/Sharing/SharingManager.php

@@ -698,7 +698,7 @@ private function validateInteraction(ShareAccessContext $accessContext, Share $s
 		$action = new ShareAction(null, array_values(array_map(static fn (SharePermission $permission): string => $permission->class, $share->getEnabledPermissions())));
 
 		$usersToCheck = [];
-		if ($share->owner->instance === null && ($ownerUser = $this->userManager->get($share->owner->userId)) !== null) {
+		if ($share->owner->instance === null && ($ownerUser = $this->userManager->get($share->owner->userId)) instanceof IUser) {
 			$usersToCheck[] = $ownerUser;
 		}