feat: add null type support

[tdakkota]

No description provided.

[codecov]

Codecov Report

Merging #294 (44b4979) into main (71f311a) will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
+ Coverage   66.87%   66.91%   +0.03%     
==========================================
  Files         116      116              
  Lines        7696     7717      +21     
==========================================
+ Hits         5147     5164      +17     
- Misses       2207     2211       +4     
  Partials      342      342              

Impacted Files	Coverage Δ
internal/ir/faker.go	0.00% <0.00%> (ø)
internal/ir/json.go	83.42% <0.00%> (-0.94%)	⬇️
jsonschema/schema.go	100.00% <ø> (ø)
gen/generics.go	53.12% <100.00%> (+2.57%)	⬆️
gen/schema_gen.go	78.21% <100.00%> (ø)
gen/schema_gen_primitive.go	88.23% <100.00%> (+0.30%)	⬆️
internal/ir/primitive.go	66.66% <100.00%> (+1.44%)	⬆️
internal/ir/template_helpers.go	74.41% <100.00%> (+0.60%)	⬆️
jsonschema/parser.go	80.87% <100.00%> (+0.32%)	⬆️

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