feat(middleware): allow passthrough response without enforcing explicit return

[kricsleo]

resolves #1172

This PR enables middleware to pass through the response from other middlewares or handler without enforcing an explicit return next().

This improves DX, especially when the user doesn't care about the response and is likely to forget to return after calling next().

Before:

app.use(async (event, next) => {
  // before...
  const result = await next()
  // after...

  // ↓ this is likely to be forgotton.
  return result
})

After:

app.use(async (event, next) => {
  // before...
  await next()
  // after...

  // no needs to return
})

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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