fix: return empty int ndarray instead of None for class_id on empty VLM parse

[YousefZahran1]

What

On empty-parse paths in two VLM parsers, class_id was returned as None instead of a zero-length ndarray of dtype int, inconsistent with every other exit point in these functions and with the convention established by from_google_gemini_2_5, from_qwen_2_5_vl, and from_qwen_3_vl.

Affected code paths

Function	Condition	Before	After
from_paligemma	regex finds no matches	None	np.empty((0,), dtype=int)
from_google_gemini_2_0	JSONDecodeError	None	np.empty((0,), dtype=int)
from_google_gemini_2_0	empty bounding-box list	None	np.empty((0,), dtype=int)

Why it matters

Callers that do class_id.shape, len(class_id), or class_id[mask] immediately after parsing will get an AttributeError / TypeError whenever zero detections are returned — a very common path in production (frame with nothing to detect). Downstream Detections.from_vlm(...) pipelines hit this at runtime while unit tests may pass because they check happy paths.

Changes

• src/supervision/detection/vlm.py — 3 one-line fixes (no logic change)
• tests/detection/test_vlm.py — updated 10 test expectations to match the new contract

All 88 tests in tests/detection/test_vlm.py pass.

Fixes #2219

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78%. Comparing base (87b1b86) to head (7e57e2c).
⚠️ Report is 4 commits behind head on develop.

❌ Your project check has failed because the head coverage (78%) is below the target coverage (95%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2239   +/-   ##
=======================================
  Coverage       78%     78%           
=======================================
  Files           66      66           
  Lines         8347    8347           
=======================================
  Hits          6475    6475           
  Misses        1872    1872           

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[YousefZahran1]

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