monaco-graphql: Unexpected EOF with comments-only content

A user [reported a UI bug](https://github.com/jonathanawesome/graphiql-prototype/issues/63) in the GraphiQL design prototype I've been working on that's related to `monaco-graphql`.

When the model value consists only of comment lines, we get a syntax error:

This is reproducible in the [monaco-graphql-react-vite](https://stackblitz.com/edit/monaco-graphql-react-vite?file=src/App.tsx) example.

While it's simply a little annoying to see the error in the editor, the impact is heavy when attempting to parse the comments-only value because it errors out.

Ideally, a comments-only value wouldn't produce an error during parsing.

I can work around this in the prototype, so this is low priority.