Improve handling of 'empty' values for ChoiceField

The empty value defaults back to '' (for backwards-compatibility) but
is changed automatically to None for ModelSerializers if the `null`
property is set on the db field.
Drop u'' prefix for python 3.x compatibility
Merge pull request #1214 from diox/choicefield-typed

Improve handling of 'empty' values for ChoiceField