Amazon expects and empty string for NoDevice if it is set

Was unable to successfully create ami's with packer that had no ephemeral storage defined.  After reviewing the docs at http://docs.aws.amazon.com/cli/latest/reference/ec2/create-image.html and looking at how boto handles block devices, determined that EC2 expects an empty string if NoDevice is set.

Have successfully build ami's without ephemeral devices using this fix.