Update to artist credit handling

tests/integration.rs

@@ -53,6 +53,7 @@ impl MetadataProvider for MockProvider {
                 }],
                 duration_ms: Some(413_000),
                 genres: vec!["Progressive Rock".into()],
+                secondary_artists: vec![],
             })
         } else {
             Err(shanty_tag::TagError::Other("not found".into()))