fix(abacus): omit unavailable MD force labels

[njzjz-bot]

Fixes #1000.

Represent missing ABACUS MD FORCE columns as unavailable labels instead of fabricated zero arrays.

Tests: cd tests && python -m unittest test_abacus_md.TestABACUSMD

Why existing tests missed it: All existing ABACUS MD fixtures contained FORCE columns; neither the helper nor full LabeledSystem path covered a no-force dump.

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh

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📥 Commits

Reviewing files that changed from the base of the PR and between 0416b54 and 85a867f.

📒 Files selected for processing (2)

• dpdata/formats/abacus/md.py
• tests/test_abacus_md.py

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[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

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✅ 2 untouched benchmarks

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_{Comparing njzjz-bot:fix/issue-1000 (85a867f) with master (0416b54)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.87%. Comparing base (0416b54) to head (85a867f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1024   +/-   ##
=======================================
  Coverage   86.87%   86.87%           
=======================================
  Files          89       89           
  Lines        8266     8269    +3     
=======================================
+ Hits         7181     7184    +3     
  Misses       1085     1085           

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