x/pkgsite: omit "Output" section for examples with no output

Here is an example with no output:
https://pkg.go.dev/github.com/vmg/backoff#example-Ticker

The code is here:
https://go-mod-viewer.appspot.com/github.com/vmg/backoff@v1.0.0/example_test.go#L21

There are no // Output comments. 

The display on pkg.go.dev should not show an "Output:" section.