Avoid Defaulted AssertNotNull when there are no variables to pass with query

[michaelschlies]

gql/gql-ratpack/src/main/java/gql/ratpack/GraphQLHandlerUtil.java

Line 67 in 7982dc4

Map<String,Object> variables = (Map<String,Object>) payload.get("variables");

This line should look more like
Map<String,Object> variables = (Map<String,Object>) payload.getOrDefault("variables", new HashMap<String, Object>()) to avoid an AssertNotNullException in GraphQL handling.

[mariogarcia]

I'm fixing this now. I'm taking your changes and adding a test to it. Thanks @michaelschlies ! I'm releasing a new version during the weekend.