Fix for #664, crashed due to ig.Attachements being 0

This is a fix for #664
By setting the ID of this resource to an empty string, you will create a dangling resource. The Internet Gateway will still exist, but after resetting the ID Terraform will think it doesn't and will drop any references to this resource from it's state file.

What you actually want to do here is clear the vpc_id. If you set the vpc_id to an empty string, the resource will understand that the Internet Gateway still exists, but is not attached to a VPC. This way you can keep managing the Internet Gateway and attach it to another VPC  (or choose to properly delete the resource) if you want/need to.