alternator-test: add tests for empty strings in keys
alternator-test: add tests for empty strings in keys

Empty string/binary blob values are not accepted by DynamoDB,
and we should follow suit.

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