fix: support fields with ,omitempty but no name

[danielgtaylor]

Based on a comment by @lazharichir this was an annoying bug where json:",omitempty" resulted in blank names in Huma-generated schemas. This is actually allowed by the standard library and means to just re-used the field name, so Huma now follows that same logic.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.29%. Comparing base (f160410) to head (2964069).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #341   +/-   ##
=======================================
  Coverage   95.29%   95.29%           
=======================================
  Files          19       19           
  Lines        2827     2828    +1     
=======================================
+ Hits         2694     2695    +1     
  Misses         97       97           
  Partials       36       36           

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