[BugFix] Don’t compute reorder threshold when there are no attention groups

[hl475]

Purpose

This PR fixes a startup crash in the v1 runtime for attention‑free models (e.g., Terratorch) introduced after #27809. The engine unconditionally computed the batch reorder threshold even when no attention backends were created, leading to:

TypeError: reduce() of empty iterable with no initial value

from the nightly run (https://buildkite.com/vllm/ci/builds/37041/steps/canvas?sid=019a386d-1b25-4c07-9a9b-085c1e07ea05, https://buildkite.com/vllm/ci/builds/37041/steps/canvas?sid=019a386d-1b26-4f42-b55f-f0125da20368)

This PR (1) skip the calculation when there are no attention groups, and (2) make calculate_reorder_batch_threshold() defensive by resolving an empty list to None.

Test Plan

CI

Test Result

Basic Models Tests (Extra Initialization) 1 + Basic Models Tests (Extra Initialization) 2
https://buildkite.com/vllm/ci/builds/37052/steps/canvas?sid=019a3901-5ee6-45ba-bace-2ccb858b53a1
Basic Models Tests (Initialization)
https://buildkite.com/vllm/ci/builds/37052/steps/canvas?sid=019a3901-5ee5-48b7-ac1d-0cf8db1b054d

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• The test plan, such as providing test command.
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[LucasWilkinson]

Thanks for the contribution! Overall LGTM, left a couple nits

[hl475]

Thanks @LucasWilkinson for reviewing!

I just updated this PR to address the comments, please take another look!