Signing in on the measure page retains the large signin CTA

[samthor]

Describe the bug
When you sign in on https://web.dev/measure, the large "Sign In With Google" button in the middle of the page sticks around until you navigate back/forwards.

Note that signing out where the button is hidden, correctly makes the button visible again.

To Reproduce
Go to measure page, sign in, see button remain, navigate in/out, see that button is hidden.

Expected behavior
Sign in button disappears.

Screenshots

[samthor]

Returning "" from the render() method in our LitElement subclasses—turns out it was silently not being rendered. This is solved by a recent PR in lit-element but is yet to be released.

I'm sending a PR that replaces our return "";'s with something that returns an empty template, rather than an empty string.