Improve nullable

[fabien0102]

Why

Use nullable() when this is possible, this is solving half of the case from #54 and this is a bit more semantic.

[codecov-commenter]

Codecov Report

Merging #57 (9f549f6) into main (e5e4961) will decrease coverage by 1.30%.
The diff coverage is 95.20%.

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   98.49%   97.19%   -1.31%     
==========================================
  Files           8       11       +3     
  Lines         332      463     +131     
  Branches      102      144      +42     
==========================================
+ Hits          327      450     +123     
- Misses          5       10       +5     
- Partials        0        3       +3     

Impacted Files	Coverage Δ
src/core/generateZodSchema.ts	95.45% <91.07%> (-1.75%)	⬇️
src/utils/resolveDefaultProperties.ts	95.23% <95.23%> (ø)
src/utils/resolveModules.ts	97.61% <97.61%> (ø)
src/core/generate.ts	100.00% <100.00%> (ø)
src/core/jsDocTags.ts	98.50% <100.00%> (-1.50%)	⬇️
src/core/validateGeneratedTypes.ts	97.43% <100.00%> (+0.06%)	⬆️
src/utils/isNotNull.ts	100.00% <100.00%> (ø)

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