Handle the lack of userHandle better in demo

[abergs]

Fix crash in usernameless login demo when authenticator doesn't return a userHandle (non-resident keys) and also guard against null userHandle before wrapping in Uint8Array, which would throw a TypeError

Re-implements #648.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.59%. Comparing base (7f16774) to head (41f7d86).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #653   +/-   ##
=======================================
  Coverage   78.59%   78.59%           
=======================================
  Files          98       98           
  Lines        2542     2542           
  Branches      423      423           
=======================================
  Hits         1998     1998           
  Misses        434      434           
  Partials      110      110           

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