Generator should skip generation for empty services

[alicejli]

A scenario came up with googleapis/google-cloud-java#10802 where https://github.com/googleapis/googleapis/blob/master/google/cloud/visionai/v1/prediction.proto contains an empty service (e.g. no RPCs).

There tests fail when trying to compile the generated library containing this proto which means the generator is generating some classes for this empty service which are not compilable. The generator instead should skip generation for an empty service.