to_primitive should always return None when value is None

[alexhayes]

If using a DecimalType and you supply a None value, when calling to_primitive it returns 'None' (string).

This PR ensures that None is returned for all fields if the value of the field is None.

[lkraider]

Not sure about the use case here, when you serialize a Model the fields that have None will be in the output (unless serialize_when_none is False). This works today:

def test_none_to_primitive():
    class Foo(Model):
        i = IntType()
        s = StringType()
        d = DecimalType()
        u = UUIDType()

    class Bar(Foo):
        class Options:
            serialize_when_none = False

    f = Foo()
    assert f.serialize() == {'i': None, 's': None, 'd': None, 'u': None}
    b = Bar()
    assert b.serialize() == {}

[lkraider]

I don't see the use case here. Please provide some tests that identify the usefulness of this code. Please see my previous comment on the PR for a counter example why I think this isn't needed.

[lkraider]

Closing, but please open a new Issue or PR in case you think this is still needed.