fix: union properties can be optional & nullable

[tvillaren]

Why

This PR fixes #65
Feel free to refuse, my knowledge of the source code is very limited and I might not have all the perspective needed for this edit!

[codecov-commenter]

Codecov Report

Merging #66 (ea35e24) into main (e5e4961) will decrease coverage by 1.23%.
The diff coverage is 95.59%.

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
- Coverage   98.49%   97.25%   -1.24%     
==========================================
  Files           8       11       +3     
  Lines         332      474     +142     
  Branches      102      157      +55     
==========================================
+ Hits          327      461     +134     
- Misses          5       10       +5     
- Partials        0        3       +3     

Impacted Files	Coverage Δ
src/core/generateZodSchema.ts	95.54% <91.93%> (-1.66%)	⬇️
src/utils/resolveDefaultProperties.ts	95.23% <95.23%> (ø)
src/utils/resolveModules.ts	97.87% <97.87%> (ø)
src/core/generate.ts	100.00% <100.00%> (ø)
src/core/jsDocTags.ts	98.50% <100.00%> (-1.50%)	⬇️
src/core/validateGeneratedTypes.ts	97.50% <100.00%> (+0.13%)	⬆️
src/utils/isNotNull.ts	100.00% <100.00%> (ø)

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[fabien0102]

Looks good to me! Thanks for this fix! 💯