fix: non required argument can be undefined if it is not provided at …

[n1ru4l]

…all and has no default value.

closes https://github.com/sikanhe/gqtx/issues/34

type Query {
  foo(a: String): Boolean
}

query foo {
  foo
}

For the following query on the given schema, the actual runtime type in the resolver for the a argument is undefined. It is only null if the argument is explicitly set to null, either via a default argument, or variable.

I guess this makes sense as in your resolvers you might have a database calls that updates a value that is either a string or null. If the argument is undefined that means the value must not be updated.

[sikanhe]

@n1ru4l Makes sense