Related to #118243: we currently have a check on the Dart side for a null response from a Host API call, which means that there was no handler set up, and we error accordingly. We don't have any comparable handlers for Flutter API.

As part of, or after, fixing #118243 we should also check for a null response and convert that to an error about not being able to establish communication.

We should also add an integration test that deliberately calls a Flutter API that doesn't have a Dart hander set up (we can define a new, minimal dummy Flutter API in the test pigeons/ file that's explicitly documented as never being set up on the Dart side).