fix(Preview): Fix sharding of the PreviewMigrationService

Signed-off-by: Carl Schwan <carl@carlschwan.eu>

Author: CarlSchwan

lib/private/Preview/PreviewMigrationService.php

@@ -224,6 +224,7 @@ private function folderHasChildren(int $storageId, string $path): bool {
 		$qb->select('fileid')
 			->from('filecache')
 			->where($qb->expr()->eq('parent', $qb->createNamedParameter((int)$folderId)))
+			->andWhere($qb->expr()->eq('storage', $qb->createNamedParameter($storageId)))
 			->setMaxResults(1);
 		$cursor = $qb->executeQuery();
 		$hasChild = $cursor->fetchOne() !== false;