fix: return empty slice if requested_scope or audience is null

[jonas-jonas]

Related issue(s)

Fixes #3655

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
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  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bfc05d0) 76.06% compared to head (254547b) 76.22%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3711      +/-   ##
==========================================
+ Coverage   76.06%   76.22%   +0.15%     
==========================================
  Files         134      134              
  Lines       10142    10210      +68     
==========================================
+ Hits         7715     7783      +68     
  Misses       1910     1910              
  Partials      517      517              

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[aeneasr]

Do this in consent as well?

Or just override the MarshalJSON method?