DynamoDB Query pager broken

[GrahamCampbell]

After the latest update, using ->getItems(false) on a query results in an error from DynamoDB: The provided starting key is invalid. Presumably this was caused by #1794.

[GrahamCampbell]

The problem is that lastEvaluatedKey gets set to an empty object, not null, when we're at the last page.

[jderusse]

I'm on it