Add a default values for rpc with empty message

### 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:

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

To something like:
```python
answer = TestStub().call()
```

No files or tests are identified in the issue. Start by locating the generated Python RPC method signatures and the current handling of google.protobuf.Empty or fieldless messages; done means empty-message RPCs can be called without an argument while existing explicit-message calls continue to work.

Written by the indexing model from the issue text.