[Bugfix][DSv4][SM120] Skip empty sparse-MLA prefill chunks

[ormandj]

Purpose

FULL_AND_PIECEWISE CUDA-graph padding can create a trailing SM120 sparse-MLA prefill chunk with query_start == query_end. Passing that chunk to FlashInfer raises RuntimeError: cannot reshape tensor of 0 elements into shape [0, -1] and terminates EngineCore.

Skip empty query ranges. Non-empty chunks are unchanged.

Duplicate check

No other open PR addresses this empty-query guard.

Tests

• uv run --active --no-sync python -m pytest tests/kernels/attention/test_flashmla_sparse.py -k cudagraph_padding_chunks -v: 1 passed, 5 deselected.
• uv run --active --no-sync pre-commit run --files vllm/models/deepseek_v4/nvidia/flashinfer_sparse.py tests/kernels/attention/test_flashmla_sparse.py: passed.
• SM120 TP=2: a 14-token prompt selecting the 15-token graph returned HTTP 200 four times; the pod remained Ready with zero restarts.

Model evaluation

Not run. The skipped ranges contain no query tokens.

Created with AI assistance.

[github-actions]

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