Description

In the latest version of the spec, empty types (input objects, object types) must be represented without curly braces, i.e the following is valid:

type Query 
# below only here to make the sdl valid 
extend type Query {
  foo: String
}

while the following technically isn't (note the {}):

type Query {}
# below only here to make the sdl valid 
extend type Query {
  foo: String
}

Because of historical reasons, graphql-java supports parsing of both syntaxes. However its AstPrinter prints the spec invalid form. While this is ok in systems where graphql-java's GraphQL parser is the only one in use, it fails in poly-parser systems where stricter (albeit spec-compliant) parsers might fail to parse graphql-java printed SDL output.

Approach

This PR updates the AstPrinter to be spec-compliant, i.e serialize empty types, empty selection sets (although should never happen) without the {}.

Follow ups

If there is interest, I can follow up with another PR that (while breaking backwards-compatibilty) will make the parser spec compliant as well, i.e only allow the form without braces {}.