Always set req.url to a valid value

[plumdog]

Describe the bug
The following
https://github.com/danielcondemarin/serverless-next.js/blob/8da92e2d8e23e361c24904e4579e9d4e02845126/packages/next-aws-lambda/lib/compatLayer.js#L15-L18
may set req.url to be an empty string - because event.requestContext.path will be '' (or will possibly be the stage name, with no trailing slash?) if you access AWS API Gateway at apidomain.com/stageName with no trailing slash.

To Reproduce
Either set up API gateway running things in Lambda (probably not worth the hassle) or create a test for this behavior (I intend to open a PR that does this)

Expected behavior
req.url should be set to a valid value

Additional context
See vercel/next.js#10953