fix(validator): allow nullable variables for nonnull args with default

[lbarthon]

The validation was failing for arguments that are defined as required, but have a default value, preventing a non-required argument to override their value, while it should work as expected.

Given the tests are imported from JS, I'm not sure how to add a new one there. Here's an example schema and query to reproduce this:

type Query {
  user(username: String! = "foo"): User
}

type User {
  username: String!
}

query Foo($username: String) {
  user(username: $username) {
    username
  }
}

I have:

• Added tests covering the bug / feature
• Updated any relevant documentation

[lbarthon]

Hey @StevenACoffman! Would you mind having a look at this?
We rely on that for gqlgen, would be great to have it bumped there too, once we land this 😁

[coveralls]

coverage: 87.549% (-0.03%) from 87.575%
when pulling 6bd0682 on lbarthon:lbarthonet/default-value-args
into 1bb503e on vektah:master.