Use schema unification for item lists and improve support for empty item lists

[mdekstrand]

This fixes and simplifies support for empty item lists by more thoroughly using Arrow schemas and schema unification, removing some special-case logic for empty item lists, and adding tests for the case where all lists in an item list collection are empty (#1051).

Closes #1051.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.69%. Comparing base (2b50f02) to head (ede2ed4).

Files with missing lines	Patch %	Lines
src/lenskit/data/collection/_base.py	75.00%	3 Missing ⚠️
src/lenskit/data/items.py	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1053      +/-   ##
==========================================
+ Coverage   89.67%   89.69%   +0.01%     
==========================================
  Files         223      182      -41     
  Lines       15476    13047    -2429     
==========================================
- Hits        13878    11702    -2176     
+ Misses       1598     1345     -253     

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