The CLN API is rather strict about the fact that we should skip
providing a field whenever it is empty. Checking for `is_none` would
still include empty arrays.
Changelog-Fixed cln-rpc: Optional empty arrays will no longer be serialized in requests
So far we were papering over the actual error with a generic string
that isn't very useful. Now we decode the error and forward it through
the grpc layer as well (though still as a string).
