fix(gen): add omitempty tag if field is optional

[tdakkota]

For #660.

[codecov]

Codecov Report

Base: 73.73% // Head: 73.77% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (c5f0517) compared to base (ffa32a1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #666      +/-   ##
==========================================
+ Coverage   73.73%   73.77%   +0.04%     
==========================================
  Files         171      171              
  Lines       11995    12012      +17     
==========================================
+ Hits         8844     8862      +18     
  Misses       2682     2682              
+ Partials      469      468       -1     

Impacted Files	Coverage Δ
gen/ir/type.go	76.61% <ø> (ø)
gen/ir/field.go	96.29% <100.00%> (+0.98%)	⬆️
gen/templates.go	82.14% <0.00%> (-0.60%)	⬇️
gen/gen_contents.go	72.78% <0.00%> (+0.68%)	⬆️

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