Fix run_graphql_field with missing query string on dummy query

[ryanseys]

Bug: The dummy_query object created as part of run_graphql_field doesn't set a query string, which results in a query_context object that has validation errors.

Later when object_type.wrap(inner_object, query_context) is called with that query_context, it checks that the field is authorized.

If as part of checking self.authorized? on an object, we first ensure that the context is valid i.e. that context.errors.empty?, we will find that it isn't because of this missing query string.

Even if this query string isn't used, the GraphQL::Query object should be valid, in case there are validations that are done on these objects in a client's code. This is what we are doing in our code, which led me to this bug.

This PR sets a dummy query string value to fix this issue, and adds a simple self.authorized? check which ensures that the context is valid i.e. that context.errors.empty?

[ryanseys]

Please take another look 😄

[rmosolgo]

Thanks for the fix!