Reduce the verbosity of generated Java/Kotlin

[stuartwdouglas]

There are some optimisations we could make here, a bit one would be not generating empty request / response objects, and generating methods with no parameters.

e.g. instead of generating

class EmptyVerbRequest{}
interface VerbClient {
 void call (EmptyVerbRequest req);
}

We could just generate:

interface VerbClient {
 void call ();
}

We need to make sure that adding additional optional parameters to the schema does not break compilation though, so if we are doing this optimisation we also need to allow for generating both methods if the request object only has optional params, e.g.:

class EmptyVerbRequest{ Optional<String> optinalParam;}
interface VerbClient {
 void call (EmptyVerbRequest req);
 void call();
}

This means that if the empty body is ever expanded with an optional parameter existing code will continue to work.