[BUGFIX][NIXL] quick fix for 'assert self.connector_worker is not None' in get_kv_connector_stats

[xuechendi]

Purpose

Propose a quick fix which occurs after #26046

vllm/tests/v1/kv_connector/nixl_integratio/run_accuracy_test.sh

File "/workspace/vllm/vllm/v1/engine/core.py", line 328, in step
engine_core_outputs = self.scheduler.update_from_output(
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/workspace/vllm/vllm/v1/core/sched/scheduler.py", line 926, in update_from_output
stats = self.connector.get_kv_connector_stats()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/workspace/vllm/vllm/distributed/kv_transfer/kv_connector/v1/nixl_connector.py", line 244, in get_kv_connector_stats
assert self.connector_worker is not None

Test Plan

Test Result

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• 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.
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[gemini-code-assist]

Code Review

This pull request correctly fixes an AssertionError in get_kv_connector_stats within the NixlConnector. The error occurs when this method is called on a scheduler-side connector, where self.connector_worker is expected to be None. The change replaces the assertion with a conditional check, which is the right approach to handle this case gracefully. The fix is well-targeted and effectively resolves the bug.