[Bugfix]: Fix final_res_batch list index out of range error

[chaunceyjiang]

Purpose

Fix https://buildkite.com/vllm/ci/builds/24047/steps/canvas?jid=01980fe5-71cd-4800-b119-932abd62ee98

Test Plan

Test Result

vllm serve Qwen/Qwen2.5-VL-3B-Instruct --max-model-len 2048  --max-num-seqs 5  --enable-prompt-tokens-details

curl -X POST -H 'Content-Type: application/json' -d '{"prompt_embeds": []}' --insecure http://localhost:8000/v1/completions

{"id":"cmpl-cf6bf8ab61894062928739ad1962f141","object":"text_completion","created":1752671527,"model":"Qwen/Qwen2.5-VL-3B-Instruct","choices":[],"service_tier":null,"system_fingerprint":null,"usage":{"prompt_tokens":0,"total_tokens":0,"completion_tokens":0,"prompt_tokens_details":null},"kv_transfer_params":null}

(Optional) Documentation Update

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Code Review

This pull request correctly fixes a list index out of range error that occurs when an empty list of prompts is provided. The change prevents accessing final_res_batch[0] when the list is empty. I've added one critical comment pointing out a related NameError that can occur under the same conditions, with a suggestion on how to resolve it to make the function fully robust.

[chaunceyjiang]

@DarkLight1337 PTAL.

[chaunceyjiang]

Note: final_res is an internal variable from the for-loop above. If final_res_batch is an empty list ([]), then final_res will not be initialized.