fix: base64 schemas endpoint

[toonvanstrijp]

Related issue(s)

#1529

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
  [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

Merging #1918 (060d216) into master (7faf83a) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
- Coverage   75.25%   75.23%   -0.02%     
==========================================
  Files         292      292              
  Lines       15175    15180       +5     
==========================================
+ Hits        11420    11421       +1     
- Misses       2946     2948       +2     
- Partials      809      811       +2     

Impacted Files	Coverage Δ
schema/handler.go	82.53% <60.00%> (-1.95%)	⬇️
persistence/sql/persister_courier.go	88.33% <0.00%> (-3.34%)	⬇️

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