regression: publicPath can't be false

[joshwiens]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

The webpack config validator does not allow publicPath: false. undefined is allowed, but not false. This change thus makes it impossible to cause webpack_public_path to be used.

If the current behavior is a bug, please provide the steps to reproduce.

See: #145

What is the expected behavior?

If this is a feature request, what is motivation or use case for changing the behavior?

Please mention other relevant information such as your webpack version, Node.js version and Operating System.