an empty configuration file and then bootstraps.

- Except when using the new init command, it is now an error to run
  buildout without a configuration file.

- 75607: buildout should not run if it creates an empty buildout.cfg


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@75710 62d5b8a3-27da-0310-9561-8e5933582275