DynamoDb: fix ToJSON instance of UpdateTable

The [AWS DynamoDb API for UpdateTable](http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_UpdateTable.html#API_UpdateTable_RequestSyntax) requires that the `GlobalSecondaryIndexUpdates` parameter is "an array of **one or more** global secondary indexes...". The generic `ToJSON` instance for lists encodes empty lists as empty arrays. This patch fixes the `ToJSON` instance of `UpdateTable` to conform with the DynamoDb API.
I think it's a bit silly (of Amazon) that Amazon makes these restrictions.