Function generate_audacity_macro 

fn generate_audacity_macro(
    output_dir: &str,
    tag: &str,
    layer_files: &[(String, String)],
    show: &str,
    season_title: Option<&str>,
    episode_title: Option<&str>,
    artist: &str,
) -> Result<Option<PathBuf>>

generate_audacity_macro(...) — None when there is no Macros directory.