Ensure robust handling of XML responses when initiating multipart
uploads. Check for the existence of required nodes before access,
and throw an exception if the XML is empty or malformed.
Refs: https://github.com/scylladb/scylladb/issues/24676
Closes scylladb/scylladb#24990
(cherry picked from commit 342e94261f)
Closes scylladb/scylladb#25054