Add a default values for rpc with empty message

[AdrienVannson]

The Problem

Sometimes, it is needed to declare RPCs that take no parameter. However, it is not possible in gRPC. In such a situation, it is needed to either create an empty message type, or to use google.protobuf.Empty ( https://stackoverflow.com/questions/29687243/protobuf-rpc-service-method-without-parameters ).

However, this is not convenient for the final user: an empty message need to be created and passed to the method each time. It also makes it necessary to add potentially long imports.

The Ideal Solution

To make this easier, it would be better to add a default value for the parameter containing the request when it is possible. I see two possibilities:

• Add the default value only for google.protobug.Empty
• Add the default value for all the messages who have no field at all

I think the second solution would be better as it is common to define empty messages in such situations. I can make a PR if you agree, I already have a working version locally.

The Current Solution

Currently, it is necessary for the user to import by hand from betterproto.lib.std.google.protobuf import Empty or the custom message, which is not convenient.

The feature would turn:

from betterproto.lib.std.google.protobuf import Empty
answer = TestStub().call(Empty())

To something like:

answer = TestStub().call()

[Gobot1234]

Yeah I think this was the behaviour for v1.2.5 and I can agree it's helpful. Might be also useful to perform implicit returns for Empties as well