Restore helptext behavior for erased argument names + tests

[brentyi]

#252 reverted the helptext grouping behavior discussed in #183 for when tyro.conf.arg(name="") is used. I restored this. The groupings that motivated #252 should be unaffected.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.86%. Comparing base (971c839) to head (3648cd0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #253   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          28       28           
  Lines        2246     2246           
=======================================
  Hits         2243     2243           
  Misses          3        3           

Flag	Coverage Δ
unittests	99.86% <100.00%> (ø)

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