Use nil_values instead of null_character in IO operations

[cnpryer]

Closes #666

Updates null_character defaults used in csv reading operations to use nil instead of "NA". See #667 (comment)

Updates usage of null_character to null_values allowing multiple missing value interpretation in IO operations. This also changes the default from "NA" to nil.

Result:

iex(8)> Explorer.DataFrame.load_csv("name,abbreviation\nNamibia,NA")
{:ok,
 #Explorer.DataFrame<
   Polars[1 x 2]
   name string ["Namibia"]
   abbreviation string ["NA"]
 >}

[philss]

It makes sense to me 👍

I added some suggestions.

[cnpryer]

I didn't use "nil" in Rust since I figured it's Elixir-specific (I'm very new to Elixir). I can change that as well if you'd like.

[josevalim]

💚 💙 💜 💛 ❤️