Join reports identity on root value change

```rs
fn join_map_root_value_false_identity() {
    let mut dst = PathMap::<u64>::new();
    let mut src = PathMap::new();
    src.insert(b"", 7);
    let status = dst.write_zipper().join_map_into(src);
    assert_eq!(dst.val_at(b""), Some(&7));
    assert_eq!(status, AlgebraicStatus::Element);
}
```