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()
```

issue 中没有确定任何文件或测试。首先定位生成的 Python RPC 方法签名，以及当前对 google.protobuf.Empty 或无字段消息的处理；完成的标准是：无需传入参数即可调用空消息 RPC，同时现有的显式消息调用继续正常工作。

由索引模型根据 Issue 内容生成。