[CI Failure] Fix test_enabled_custom_ops with empty ops

[mgoin]

Purpose

Fix failing "Model Executor Test" with an issue that was introduced by #24542

[2025-09-23T00:42:31Z]         for op in self.custom_ops:
[2025-09-23T00:42:31Z] >           if op[0] not in {'+', '-'} and op not in {'all', 'none'}:
[2025-09-23T00:42:31Z] E           IndexError: string index out of range
[2025-09-23T00:42:31Z]
[2025-09-23T00:42:31Z] /usr/local/lib/python3.12/dist-packages/vllm/config/compilation.py:491: IndexError

[2025-09-23T00:42:31Z] FAILED model_executor/test_enabled_custom_ops.py::test_enabled_ops[-0-False-ops_enabled0-True] - IndexError: string index out of range

https://buildkite.com/vllm/ci/builds/31919/steps/canvas?jid=019973de-a941-4e89-8d50-c2803680fbe2

Test Plan

Test Result

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly fixes a crash in CompilationConfig.__post_init__ caused by an empty string in the custom_ops list. The added check is appropriate and resolves the reported CI failure. I've noticed that the custom_op_log_check method in the same file has a similar vulnerability where it iterates over custom_ops without checking for empty strings, which could also lead to an IndexError. While this is outside the scope of the current changes, I recommend addressing this in a follow-up to improve the robustness of the code.

[mgoin]

Resolved by #25429