Conditionally exclude filters

# Issue

Some GraphQL APIs don’t handle optional filters properly — when a filter is not needed, it should be **omitted** from the query. Instead, we’re forced to rebuild the query string if we don’t need that filter.

I want to reuse a query like this:

```graphql
query($page: Int!, $size: Int!, $categoryId: String) {
items(page: $page, size: $size, filter: { category: { eq: $categoryId } }) {
id
}
}
```

If `categoryId` is not provided, it should be removed from the query entirely, not passed as null.

# Request

Is there a better way to handle this that I’m missing?

It would be helpful if GraphQLRequest could:

* Allow us to conditionally include/exclude filters based on the variable values.
* Provide a way to compose queries without having to rebuild them from scratch each time.