`cln_grpc`: How to use default behavior of `exposeprivatechannels`?

# Context

The `invoice`-method in the JSON-rpc has a `exposeprivatechannels`-field.
If this field is not specified Core Lightning will only include private channels in the invoice if no public channels are available.

I'd like to get the same behavior using `cln_grpc`. In this case the `exposeprivatechannels`-field which expects a vector of short-channel-id's. If the vector is empty, the invoice will not contain any private channels.