Option to allow empty ranges to be represented as None/null

[devanubis]

Hi, would you accept a PR to add an option represent_empty_as_null to RangeField so that to_representation() returns None when value.isempty

Our main use-case for this is that it would be a bit simpler for consumers of our API to just handle a null value for an absent/empty range, rather than having to process {"empty": true} 

We don't need to be able to serialise incoming null into an empty Range, but if you think that might be consistent/useful then we could add it at the same time.