fix: validation for donation ticket price

[maze-runnar]

Fixes fossasia/open-event-frontend#7816

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[mariobehling]

Please fix Circle-CI tests before merging. Thanks

[maze-runnar]

Please fix Circle-CI tests before merging. Thanks

yes, fixing tests.

[codecov]

Codecov Report

Merging #8219 (e0c82cf) into development (45f6f9e) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@               Coverage Diff               @@
##           development    #8219      +/-   ##
===============================================
- Coverage        65.74%   65.73%   -0.01%     
===============================================
  Files              305      305              
  Lines            15810    15813       +3     
===============================================
+ Hits             10394    10395       +1     
- Misses            5416     5418       +2     

Impacted Files	Coverage Δ
app/api/custom/orders.py	67.00% <33.33%> (-1.05%)	⬇️

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