Adds TestContract_MetadataGetPayloadsUseZeroRecordsInsteadOf404: empty
guest/docker metadata maps must serialize as {} (never null) and a
missing resource must return a 200 zero record echoing the requested ID
(never a 404). This is the proof companion to the
metadata_handlers_shared.go consolidation in the previous commit — it
was authored with that change but lost to a shared-index race at commit
time.
rcourtman