[now dev] Supply empty default `now.json` if one does not exist (#2231)
[now dev] Supply empty default `now.json` if one does not exist (#2231)

This forces all the files to be served as static assets using
`@now/static`, rather than having a special branch that dishes
out to `serve-static`. This is better because it keeps the logic
consistent with when a `now.json` is supplied vs. when it is not,
and allows the code to be simplified by removing checks for if
`getNowJson()` returned anything (previously it could return `null`,
but now that is not the case). Also allows us to remove the now-unused
`serveProjectAsStatic()` function.
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