Ensure that `pid` passed to `kill_process_tree` is `int` for `mypy` (#17051)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/v1/utils.py

@@ -134,8 +134,8 @@ def shutdown(proc: Process, input_path: str, output_path: str):
         proc.terminate()
         proc.join(5)
 
-        if proc.is_alive():
-            kill_process_tree(proc.pid)
+        if proc.is_alive() and (pid := proc.pid) is not None:
+            kill_process_tree(pid)
 
     # Remove zmq ipc socket files.
     ipc_sockets = [output_path, input_path]