fix @omit_null/@omit_empty macros for parametric types

[quinnj]

Summary

• Fix @omit_null and @omit_empty macros to work correctly with parametric types
• Previously, @omit_null struct Foo{T} ... end generated JSON.omit_null(::Type{Foo{T}}) = true which only matched when the exact type parameter matched
• Now generates JSON.omit_null(::Type{<:Foo}) = true which correctly matches all instantiations (e.g., Foo{Int}, Foo{String}, etc.)

Test plan

• All existing tests pass
• Manual verification with parametric types:

  @omit_null struct MyParametric{T}
      value::Union{T,Nothing}
  end
  JSON.json(MyParametric(1))  # should omit null fields for any T

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.26%. Comparing base (8d648bd) to head (5c9682f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/write.jl	83.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
- Coverage   90.27%   90.26%   -0.02%     
==========================================
  Files           7        7              
  Lines        1358     1366       +8     
==========================================
+ Hits         1226     1233       +7     
- Misses        132      133       +1     

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