Nullable variables field in request payload

It looks like the specification is not clear if the client provides a request without variables, does the field "variables" should be assigned "null" or "{}" or both are valid?

Context:
In the [altair](https://altair.sirmuel.design) tool, the default value is set to "{}" (without the double quotes) and in the graphql-java implementation, it's expecting a "null" value if not variables are needed. The result is, the queries are rejected on the server side, so does the server side is too stick or is it the client side which is providing the wrong value?