pw_grpc: Send response message for client streaming RPCs

`GrpcChannelOutput::Send` only sent the response DATA frame for a
RESPONSE packet when
* the payload was non-empty
* the method was unary

Client streaming methods also have an unary response, so when their
response message encodes to zero bytes, the message was dropped and
the call was completed with a Trailers-Only response.

Conformant clients reject this, e.g. 'grpc-go' fails the call with:

  rpc error: code = Internal desc = cardinality violation:
  received no response message from non-server-streaming RPC

Newly added `TestClientStreamingEchoEmptyResponse` in
`integration_test.go` verifies that.

Change-Id: I3dcc39ccd60fc0ebc993a906bef7d92933d7ddc8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/483830