A Sub-Document insertion only specifying XATTR paths no longer creates an empty JSON body

This is a regression that was introduced in MB-40126.

This Java 3.x code replicates:

String docId = docId();

coll.mutateIn(docId, Collections.singletonList(

            upsert("foo", "bar").xattr()

        ),

        mutateInOptions().storeSemantics(StoreSemantics.INSERT));

LookupInResult result = coll.lookupIn(docId, Arrays.asList(

        LookupInSpec.get("foo").xattr(),

        LookupInSpec.get("")

));

assertTrue(result.exists(1));

This is a Sub-Document insertion (ADD flag) that's only touching XATTRs, it doesn't specify the body and leaves it at whatever the server defaults to.

It passes on 6.5.0-4908 but fails on 6.6.0-7857, on the last line.

So memcached used to create an empty JSON object as the body if none was specified, and now creates the document with a null body instead.

(This change does seem like an improvement to me, but sadly will break existing released transactional code when running on 6.6.)