Removed unwanted error in `ping` and `save` operation callbacks

This is in response to a bug that appeared when I switch my application to the protobuf API. Sending ping and save commands threw errors like `{notFound: true}` which I found to be unwanted in the case of operations that do not return any data.

This very simple fix only checks to see if the operation requires a response body before making the said error and passing it to the callback.