MapperUtils dateTransformer() returns '1970-01-01' when null is provided

[dbleakley]

It looks like the dateTransforms may not be handling null or undefined input as expected.

Given the following test-case:

        expect(mapperUtils.dateTransformer(null)).to.be.undefined;

The dateTransformer returns '1970-01-01'.

Would it be reasonable to expect these transformers to return undefined in the event the input data is not truthy and/or possibly when the string is not a valid format -- moment(date_string).isValid === false?

[seanpk]

that sounds reasonable ... the "1970-01-01" only makes sense to a developer
since this function is for the mapping context, I think it makes sense for it to return undefined.

@dannylevenson - do you have an opinion?