fn default_profile_name() -> String {
        non_empty_env_var(AWS_PROFILE).unwrap_or_else(|| DEFAULT.to_owned())
    }
Get the default profile name:
1. if set and not empty, use value from environment variable ```AWS_PROFILE```
2. otherwise return ```"default"```
see https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/credentials.html.
rusoto/credential/src/profile.rs