dynamodb: Remove value if Exists set to false in Expected block

http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_ExpectedAttributeValue.html says:

> DynamoDB returns a ValidationException if:
> 
> Exists is true but there is no Value to check. (You expect a value to exist, but don't specify what that value is.)
> 
> Exists is false but you also specify a Value. (You cannot expect an attribute to have a value, while also expecting it not to exist.)