allow non-nullable first and last arguments in relay-connection-arguments-spec

[swac]

This addresses https://github.com/cjoudrey/graphql-schema-linter/issues/148.

The first argument type in a Relay connection can now be non-nullable if backward pagination is disabled (likewise for last when forward pagination is disabled). So the following types are now permitted:

type Query {
  users(last: Int!, before: String): UserConnection
}

type Query {
  users(first: Int!, after: String): UserConnection
}

The following, however, are not permitted:

type Query {
  users(first: Int!, after: String, last: Int, before: String): UserConnection
}

type Query {
  users(first: Int, after: String, last: Int!, before: String): UserConnection
}

[swac]

Thanks for the review @cjoudrey! I've updated accordingly 😄