Do not throw an exception if the etag is not set in metadata

Signed-off-by: Louis Chemineau <louis@chmn.me>

lib/private/FilesMetadata/Model/FilesMetadata.php

@@ -159,7 +159,7 @@ public function setEditPermission(string $key, int $permission): void {
 
 	public function getEtag(string $key): string {
 		if (!array_key_exists($key, $this->metadata)) {
-			throw new FilesMetadataNotFoundException();
+			return '';
 		}
 
 		return $this->metadata[$key]->getEtag();