Fix PHP Warning: Trying to access array offset on null

[happydude]

I'm assuming the array key doesn't exist when not using a resident key

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 10.48%. Comparing base (04738b7) to head (9c9c728).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #78   +/-   ##
=========================================
  Coverage     10.48%   10.48%           
- Complexity      324      325    +1     
=========================================
  Files            17       17           
  Lines          1259     1259           
=========================================
  Hits            132      132           
  Misses         1127     1127           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[restena-sw]

Indeed, "rk" is the only credential property extension defined in the webauthn-2 spec: https://www.w3.org/TR/webauthn-2/#sctn-authenticator-credential-properties-extension

It's safe to be more specific in the array check as suggested by the PR.