Properly update cluster read model with and empty check selection

[nelsonkopliku]

The's some issue when saving an empty checks selection.

This code does not correctly update

changeset =
        %ClusterReadModel{id: id}
        |> ClusterReadModel.changeset(%{
          selected_checks: checks
        })
Ecto.Multi.update(multi, :cluster, changeset)

The following code works but it might be subject to race conditions.

See https://github.com/trento-project/web/pull/420/commits/55c52e3517700847fd58b1e29e1740263a4ea648

changeset =
        ClusterReadModel
        |> Repo.get(id)
        # TODO: couldn't make it work with Ecto.Multi
        # With following line when we receive an empty list of selected checks, the readmodel does not get updated
        # %ClusterReadModel{id: id}
        |> ClusterReadModel.changeset(%{
          selected_checks: checks
        })
Ecto.Multi.update(multi, :cluster, changeset)