Response parity: TransactGet empty projection returns Item; BatchWrite key-less item returns 500 not 400

[hicksy]

Summary

Two data-plane response-parity gaps surfaced by the dynamodb-conformance suite (Dynoxide 0.9.13).

1. TransactGetItems projection matching nothing. When a ProjectionExpression selects no attribute present on the item, AWS omits Item from that response entry. Dynoxide returns the item with the key present ({ pk: ... }).
2. BatchWriteItem key-less item. A PutRequest whose item is missing the table key returns HTTP 500 InternalServerError. AWS returns 400 ValidationException ("The provided key element does not match the schema"). This is the same class of validate-before-internal-error issue as the closed TransactGetItems missing-key bug.