Fix: Cannot read property 'then' of undefined

[leepowelldev]

This PR fixes a Cannot read property 'then' of undefined error that gets thrown when this condition is met:

if (conn = navigator.connection) {
  // Don't prefetch if using 2G or if Save-Data is enabled.
  if (conn.saveData || /2g/.test(conn.effectiveType)) return;
}

prefetch is meant to return a Promise, however in this instance it returns undefined, meaning that this throws:

prefetch(entry.href, options.priority).then(isDone).catch(err => {
  isDone(); if (options.onError) options.onError(err);
});

I tried to add tests for this, but I could not figure out a way of mocking either saveData or a 2g connection - I tried multiple approaches, with none working. If anyone has any ideas then I'm happy to add the tests to cover this.

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[leepowelldev]

@googlebot I signed it!

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[leepowelldev]

@googlebot I signed it!

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[addyosmani]

Thanks, Lee!