Misleading comment in the Invoke example

Looks like it was copy/pasted from the [pubsub](https://github.com/dapr/rust-sdk/blob/b69e1f84b04858f72de4c70793cc353fcbf08529/examples/pubsub/subscriber.rs#L25) example.

_Topic `A`_ Is not returned here, only the default (which is an empty list, given `Vec` is `Default` over `T`):

https://github.com/dapr/rust-sdk/blob/b69e1f84b04858f72de4c70793cc353fcbf08529/examples/invoke/grpc/server.rs#L55-L66

Perhaps it should read something like:

```rs
/// Return an empty list of topics.
```

... but on that note, **should the trait methods provide these default behaviours** so we don't have to implement defaults for each unused method?

Start with examples/invoke/grpc/server.rs at lines 55-66 and compare the referenced pubsub/subscriber.rs comment. Then inspect the relevant trait methods to determine whether the issue is limited to correcting the misleading documentation or also requires deciding how default behaviors should be provided; done means the comment and any resulting trait behavior match the project’s decision.

Written by the indexing model from the issue text.