Fix invalid mfa validation if value was nil

[pnezis]

iex(1)> schema = [foo: [type: :mfa]]
[foo: [type: :mfa]]
iex(2)> NimbleOptions.validate([foo: nil], schema)
{:ok, [foo: nil]}

[whatyouhide]

I’m surprised that that specific check was there! Have you dug through the blame here?

[pnezis]

it's there since the first commit :-)

0ed1d7a#diff-7fb09bff37767b08039f2861a5b0deb150b2a95f29e41746ae719196f9880e70

[whatyouhide]

@josevalim any idea? I’m good with this change to be clear, just want to make sure we're not breaking something or missing something.