feat: add support for empty structs to the 2.0 format

[westonpace]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.65517% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.41%. Comparing base (9211330) to head (532d745).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...ust/lance-encoding/src/encodings/logical/struct.rs	86.76%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3499      +/-   ##
==========================================
+ Coverage   78.40%   78.41%   +0.01%     
==========================================
  Files         252      252              
  Lines       93931    94011      +80     
  Branches    93931    94011      +80     
==========================================
+ Hits        73642    73715      +73     
- Misses      17295    17305      +10     
+ Partials     2994     2991       -3     

Flag	Coverage Δ
unittests	78.41% <89.65%> (+0.01%)	⬆️

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