Allow Nullable action response

Actions can be defined to have a nullable return type like so:

type Mutation {
  authentication_refresh (
    dummy: String
  ): AuthRefresh
}

Yet returning an empty object or empty body will throw an error. Nullable return types should either explicitly be disallowed, or not throw errors

[pranshi06]

Closed via https://github.com/hasura/graphql-engine/commit/be1395de310e5e86dc7534db17e7c297f337272c

[BenoitRanque]

Fix has been merged into main, should be out in next release, thank you @pranshi06