Distinguish between undefined and null field argument changed

[n1ru4l]

Is your feature request related to a problem? Please describe.

type Dummy {
  field(foo: Boolean): String
}

and

type Dummy {
  field(foo: Boolean = null): String
}

are not the same. Within the resolver you would either get undefined or null for the foo argument value.

This does not only apply to JavaScript land, this also exists in other languages.

• See Support where Null value came from graphql-rust/juniper#183
• See Differentiate explicit null vs. not provided fields graphql-python/graphene-django#1118

Describe the solution you'd like

Setting the default value of a field argument to null should be treated as if the field argument value would be set to another value, which is ChangeType.FieldArgumentDefaultChanged.

Describe alternatives you've considered

None

Additional context

I like turtles

PR with tests: #2387