xharness provide links to empty files

### Steps to Reproduce

1. open `http://xamarin-storage/jenkins/xamarin-macios/d16-8/e30c41de32f64eca655953b17c78c1d9d70249e3/39/jenkins-results/tests/index.html`
2. click on the `Standard output` out `Standard error` of the first failure (timeout)

This is because there's no output, fine 😄

However it means
* we have to click to see there's nothing (and then click back), we're losing time
* we have to store a bunch of empty file on xamarin-storage (or anywhere else in the future). This slows down xharness.

### Expected Behavior

Some data *or* no (clickable) links, e.g. `Standard output (empty)`

That would be similar to `Test Log` which shows `TestLog (does not exist)`

### Actual Behavior

Empty page. No hint it's empty.

### Environment

CI