fix: don't return nil if code is invalid

[jonas-jonas]

Related issue(s)

Closes https://github.com/ory-corp/cloud/issues/5648

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 the approval (please contact
  [email protected]) 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.

Further Comments

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6cf7fc5) 78.38% compared to head (f5b6054) 78.34%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
selfservice/strategy/code/strategy_verification.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3662      +/-   ##
==========================================
- Coverage   78.38%   78.34%   -0.04%     
==========================================
  Files         345      345              
  Lines       23552    23518      -34     
==========================================
- Hits        18461    18426      -35     
- Misses       3695     3699       +4     
+ Partials     1396     1393       -3     

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