Parse empty string as nil

[DanielZendejas]

Hey, I had the use case where the front end might send an empty string for a numeric value.

I forked your project and added one line that would allow it to parse the empty string as nil.

Do you think this is a common scenario? If you like it then I could make a PR for this small change.

[joshdholtz]

@DanielZendejas If you want to make a PR that would be great!

[nickgartmann]

I would suggest

def cast(""), do: {:ok, nil}

as being a bit more idiomatic elixir, since you're testing for the special case of ""

[DanielZendejas]

Cool, the PR is now in #2 .