Handle module.id being null

[patrick-webs]

Fixes error "TypeError: Path must be a string. Received undefined" when
operating on minified output that had been processed by derequire, e.g.

{
  id: undefined,
  source: '"use strict";var Promise=_dereq_("./promise.js")();module.exports=Promise',
  deps: {
    './promise.js': '/Users/username/src/myproject/core/node_modules/bluebird/js/main/promise.js'
  }
}