See feedback in #2126 (comment) from review of PR #2126 which calls out that empty_call_test should test that an exception is raised if request=None and a field in the request message is marked as required.

But we're applying this test even to methods where the API requires a non-empty request, right? It would be great if we type-hinted methods sufficiently so that for methods requiring a request, the type is specified as the proper request object, and for methods with optional requests, the type is specified as a nullable request of the appropriate type. Then this macro would be applied to just the latter set of methods (and we could also test that sending request=None to the former set would raise warnings or errors).

We won't be able to change the type hint because flattened fields could be sent instead of using the request argument but we should be able to raise an exception if request=None and a field in the request message is marked as required

We could also consider adding this as part of client library AIPs if it doesn't already exist.