Fix non-nullable type with two options

[jagregory]

If you have a type with exactly two options, neither of which is null, then the null-checking logic incorrectly uses the last item in the list as the type instead of interface{}.

e.g. "type": ["string", "boolean"] becomes *bool instead of interface{} because the generation logic is assuming a two item type is only used when setting a type to null.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.28%. Comparing base (d963216) to head (d9668fc).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   76.58%   78.28%   +1.69%     
==========================================
  Files          24       24              
  Lines        1892     1515     -377     
==========================================
- Hits         1449     1186     -263     
+ Misses        354      240     -114     
  Partials       89       89              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.