* fix(execute-exchange): prune undefined variables

This allows for directives to correctly validate with variables using default values.
```graphql
query($count: Boolean = false) {
  users {
    # Fails if $count is not provided:
    # Argument "if" of non-null type "Boolean!" must not be null.
    count @include(if: $count)
    list { id name }
  }
}
```

* Add tests and remove unnecessary closure

* Fix test name and ensure it fails if the fix isn't applied

* Revert unnecessary fixture change

* Remove unnecessary mockClear()