Make optional parameters omittable

[dahlia]

According to the serialization specification, method parameters that have an optional type should can be omitted and treated as None if omitted. As this behavior was missing from the previous PR #278, this patch fixes that.

[codecov]

Codecov Report

Merging #280 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master    #280      +/-   ##
=========================================
- Coverage   77.21%   77.2%   -0.02%     
=========================================
  Files          34      34              
  Lines        2669    2672       +3     
  Branches      144     144              
=========================================
+ Hits         2061    2063       +2     
- Misses        464     465       +1     
  Partials      144     144

Impacted Files	Coverage Δ
src/Nirum/Targets/Python.hs	88.17% <66.66%> (-0.17%)	⬇️

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