Allow schema without operations if Federation is enabled

[jmartisk]

Fixes #2110

[jmartisk]

@nicholassmith does this resolve your problem in #2110?

[nicholassmith]

I think it's a starting point, but after experimenting with manipulating the Operations when it invokes validateSchema in the GraphQLSchema::buildImpl() it's checking presence of a Query. Unfortunately that validate occurs prior to the Federation.transform which would populate the federation queries so it seems like some changes would be needed there as well.

[jmartisk]

Ah, right, I didn't notice. In that case though, that's something that would need fixing in graphql-java (https://github.com/graphql-java/graphql-java) - perhaps start a discussion there?