rust: handle google.protobuf.Empty as RPC request message

The `GetAccountForGcpRegistration` in [`google/shopping/merchant/accounts/v1/developerregistration.proto`](https://github.com/googleapis/googleapis/blob/097dd18bb9e18f389634981a9270c4cb8f768a31/google/shopping/merchant/accounts/v1/developerregistration.proto#L69) specifies `google.protobuf.Empty` as the RPC request message.

This produces a compilation error:
```
error: unused variable: `req`
--> src/generated/shopping/merchant/accounts/v1/src/transport.rs:2437:9
| 2437 | req: wkt::Empty,
| ^^^ help: if this is intentional, prefix it with an underscore: `_req`
|
= note: `-D unused-variables` implied by `-D warnings`
```

Start with google/shopping/merchant/accounts/v1/developerregistration.proto at the GetAccountForGcpRegistration definition, then inspect the generated Rust method in src/generated/shopping/merchant/accounts/v1/src/transport.rs around line 2437. Reproduce the Rust compilation with warnings denied and trace how this RPC is generated. Done means the generated client compiles without the unused-variable error.

Written by the indexing model from the issue text.