When feature flag is disabled the RegistriesResolver should return null

We include RegistriesResolver into every resolver in the GeoNodeType however it doesn't follow the state of the feature flag. The null has to be returned when the feature flag is disabled. See !100750 (comment 1135401703) for details

We should add tests which show that the resolver returns results with null for a particular data type when that data type's feature flag is off. This will also show that the resolver doesn't raise an error when the feature flag is off.

Implementation guide

1. In RegistriesResolver#resolve https://gitlab.com/gitlab-org/gitlab/-/blob/v17.8.0-ee/ee/app/graphql/resolvers/geo/registries_resolver.rb#L34-41, do return registry_class.none unless replicator_class.replication_enabled? /cc @mkozono
2. Add a test context after this line in https://gitlab.com/gitlab-org/gitlab/-/blob/v17.8.0-ee/ee/spec/support/shared_examples/graphql/geo/geo_registries_resolver_shared_examples.rb#L92 like context 'when replication is disabled' do. Stub the replication enabled feature flag to be off like stub_feature_flags(replicator_class.replication_enabled_feature_key => false) Add an example inside of that context that shows that the result is empty (look at the surrounding examples)