fix: get_node should return null on missing path

[appcypher]

Summary

rs-wnfs get_node fns in PublicDirectory and PrivateDirectory currently do not replicate the same behavior in webnative when a non-existent path segment is specified. It returns anErr(...) instead of a Ok(None) which translates to null on the js side.

This PR solves that although there is the little inconvenience that wasm-bindgen translates Ok(None) to undefined on the js side.

Test plan (required)

scripts/rs-wnfs.sh test

Closing issues

Fixes #222

[codecov]

Codecov Report

Merging #253 (e1911d1) into main (bccc0ec) will increase coverage by 0.12%.
The diff coverage is 50.00%.

❗ Current head e1911d1 differs from pull request most recent head ef8052f. Consider uploading reports for the commit ef8052f to get more accurate results

@@            Coverage Diff             @@
##             main     #253      +/-   ##
==========================================
+ Coverage   61.72%   61.84%   +0.12%     
==========================================
  Files          35       35              
  Lines        2662     2663       +1     
  Branches      655      655              
==========================================
+ Hits         1643     1647       +4     
+ Misses        566      563       -3     
  Partials      453      453              

Impacted Files	Coverage Δ
wnfs/src/private/directory.rs	71.82% <50.00%> (+0.43%)	⬆️
wnfs/src/public/directory.rs	70.27% <50.00%> (+0.90%)	⬆️

... and 4 files with indirect coverage changes

[matheus23]

LGTM! :) Just need to make newer clippy happy.