Fix internal bots by setting empty list

Change-Id: I3d7a498fbc6b45131b902104220316392d2f4de3
diff --git a/tools/run_on_app.py b/tools/run_on_app.py
index 78c8172..07de70c 100755
--- a/tools/run_on_app.py
+++ b/tools/run_on_app.py
@@ -579,7 +579,8 @@
             stderr=stderr,
             timeout=options.timeout,
             quiet=quiet,
-            cmd_prefix=['taskset', '-c', options.cpu_list] if options.cpu_list else None)
+            cmd_prefix=[
+                'taskset', '-c', options.cpu_list] if options.cpu_list else [])
       if exit_code != 0:
         with open(stderr_path) as stderr:
           stderr_text = stderr.read()