fix: allowed_top_level_claims default to nil

[fehrnah]

allowed_top_level_claims were set to nil if claim was not present

Related issue(s)

When allowed_top_level_claims are configured, they are always present in the token, with value null if they were never set.

This PR fixes this and adds a regression test

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
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Awesome find, thank you!

[codecov]

Codecov Report

Merging #3245 (7f5b9cf) into master (05bda6b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3245      +/-   ##
==========================================
+ Coverage   76.72%   76.74%   +0.02%     
==========================================
  Files         123      123              
  Lines        8846     8847       +1     
==========================================
+ Hits         6787     6790       +3     
+ Misses       1634     1633       -1     
+ Partials      425      424       -1     

Impacted Files	Coverage Δ
oauth2/session.go	80.32% <100.00%> (+0.32%)	⬆️
persistence/sql/persister_oauth2.go	82.88% <0.00%> (+0.76%)	⬆️

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[vinckr]

Hello @fehrnah
Congrats on merging your first PR in Ory 🎉 !
Your contribution will soon be helping secure millions of identities around the globe 🌏.
As a small token of appreciation we send all our first time contributors a gift package to welcome them to the community.
Please drop me an email and I will forward you the form to claim your Ory swag!