Change 'ScaffoldClient.collect_aux_var' behavior on unused module.

* Until now, calling the `collect_aux_var` method of a `ScaffoldClient`
  module that had not processed any gradients raised a RuntimeError.
* This commit changes that behavior for something less stringent:
  warn a RuntimeWarning that such a call is unexpected, but return
  a conventional, zero-valued scalar state.
* Conversely, upon receiving such states, `ScaffoldServer` ignores the
  client (i.e. it is not part of the global state's update calculation).
* The rationale behind this change is that it may happen that a client
  does not perform any optimization step, e.g. due to their DP budget
  having been saturated. In that case, we do not want the FL process
  to crash, but rather the client to be ignored as part of the update
  process.