Updated Google Cloud Function request handler to handle null or undefined req.path

[reaktivo]

The current implementation of the apollo-server-cloud-functions exported handler doesn't handle bare endpoints, that means for example, requesting https://us-central1-apollo-gcf-test.cloudfunctions.net/handler will return a 404, but accessing https://us-central1-apollo-gcf-test.cloudfunctions.net/handler/ (notice the added slash) will.

This is due to the way GCF sets request params, in which the in the first case req.path will be set to null, and in the second it will be set to "/"

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[evans]

@reaktivo Thank you so much!