alternator: fix bug in returning an empty item in a Scan
alternator: fix bug in returning an empty item in a Scan

When a Scan selects only certain attributes, and none of the key
attributes are selected, for some of the scanned items *nothing*
will remain to be output, but still Dynamo outputs an empty item
in this case. Our code had a bug where after each item we "moved"
the object leaving behind a null object, not an empty map, so a
completely empty item wasn't output as an empty map as expected,
and resulted in boto3 failing to parse the response.

This simple one-line patch fixes the bug, by resetting the item
to an empty map after moving it out.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla