Consider better CQS in mutations

[Codeweld]

When we change the system state, we shouldn't return anything. With the current implementation and usage of message handlers for GraphQL mutations, we need to return this object. We should consider using mutation resolvers, to not break CQS

Originally posted by @Codeweld in https://github.com/BitBagCommerce/SyliusGraphQLPlugin/pull/14#discussion_r727623100