provider/aws: fix panic when user_data receives nil from StateFunc

Working on master, I started seeing interface conversion errors around noon today. You can see the exact trace [here](https://gist.github.com/ryanuber/7adb3babcc90cb9a6f40).

Running even the unmodified example from the intro doc will produce this during `terraform plan`:
```
provider "aws" {
    access_key = "ACCESS_KEY_HERE"
    secret_key = "SECRET_KEY_HERE"
    region = "us-east-1"
}

resource "aws_instance" "example" {
    ami = "ami-408c7f28"
    instance_type = "t1.micro"
}
```

The error seems to originate from f26a270, and only occurs when there is no `user_data` value present in an `aws_instance` resource.