Fix from_dict for Optional values in collections

[AllanChain]

The following fixes are applied with the help of LLM agent:

• Fix DeField.__getitem__ to preserve InnerField type when unwrapping Optional inside a collection, so the generated code accesses the loop variable directly instead of indexing into it by name
• Fix Renderer.opt() to use a simple is not None check for InnerField instead of applying the datavar.get("fieldname") pattern meant for named dataclass fields

Fixes #732

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.81%. Comparing base (f09d503) to head (227f1f8).
⚠️ Report is 66 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #733      +/-   ##
==========================================
- Coverage   90.83%   90.81%   -0.02%     
==========================================
  Files          13       13              
  Lines        2040     2308     +268     
  Branches      370      449      +79     
==========================================
+ Hits         1853     2096     +243     
- Misses        124      141      +17     
- Partials       63       71       +8     

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[yukinarit]

Thanks!