This patch adds checks validating 'BatchWriteItem' requests mostly to avoid ugly fallback message.
It changes request's behaviour in case of an empty array of WriteRequests - previously such an array was ignored and whole request might succeed, now it raises ValidationException, following the documentation and behaviour of DynamoDB.
Patch includes tests in test_manual_requests (`test_batch_write_item_invalid_payload`, `test_batch_write_item_empty_request_list`) testing with several offending cases.
Fixes#23233Closesscylladb/scylladb#23878