DocumentClient: inconsistant requirement of conditional expressions vs Dynamodb, for transact writes

[seba-i]

Hi, it is frustrating that DocumentClient transactWrite forces you to use a 'conditional expression', whereas the underlying Dynamodb transactWriteItem does not.

DocumentClient TransactWrite uses class Operation, where the field expression is a required field, even though it then gets translated into a Dynamodb TransactWriteItem, whose conditionExpression is not required.

Its inconvenient to always have to use a conditional expression, eg just PUT an object into db.

Happy to submit a PR if there's agreement (to change Option.expression type to nullable String)

// workaround...
TransactWrite(
      put: Operation(
          value: {'pk': '123', 'sk': '456'},
          tableName: 'my-table',
          expression: 'attribute_not_exists(arrrgghhhh)')), // :-(