NO_ARTIFACTS = InvalidInputException

Hello,

I'm getting the error:

```
Invalid artifacts: artifact type NO_ARTIFACTS should have null output name
```

Maybe I'm missing something?
```hcl
module "build" {
source = "git::https://github.com/cloudposse/terraform-aws-codebuild.git?ref=master"
namespace = var.environment
stage = var.project
name = var.name

# https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-available.html
build_image = "aws/codebuild/standard:4.0"
build_compute_type = "BUILD_GENERAL1_SMALL"
build_timeout = 60
artifact_type = "NO_ARTIFACTS"

# These attributes are optional, used as ENV variables when building Docker images and pushing them to ECR
# For more info:
# http://docs.aws.amazon.com/codebuild/latest/userguide/sample-docker.html
# https://www.terraform.io/docs/providers/aws/r/codebuild_project.html

privileged_mode = true
aws_region = "us-east-1"
aws_account_id = var.aws_account_numbers[0]
image_repo_name = var.image_repo_name
image_tag = var.image_tag
private_repository = true

source_credential_auth_type = "PERSONAL_ACCESS_TOKEN"
source_credential_server_type = "GITHUB"
source_credential_token = var.github_access_token

buildspec = "buildspec.yml"

# Optional extra environment variables
environment_variables = [
{
name = "AWS_DEFAULT_REGION"
value = var.aws_region
}
]
}
```