Mocca should not allow operation methods in client APIs that return void

[fabiocarvalho777]

GraphQL operations (including mutations) are not allowed to not have a selection set, or even to have an empty selection set.

This restriction though is not enforced in Mocca at the moment.

The goal of this issue is to change Mocca to not allow client APIs with GraphQL operation methods returning void. Ideally, this validation should happen at client build time. But, if not possible, it is ok to do so at operation execution time (in this case at least add a TODO comment in there stating to move it to client build time).

Tests, and that sample client, should also be changed accordingly.