sysmon: fix crash when facing a process with empty commandline

BUG=chromium:745025
TEST=None

Change-Id: I6dd9790062d172c6fdfcb01a83e3e0e1c03f04e2
Reviewed-on: https://chromium-review.googlesource.com/575400
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/scripts/sysmon/proc_metrics.py b/scripts/sysmon/proc_metrics.py
index b592a95..245397f 100644
--- a/scripts/sysmon/proc_metrics.py
+++ b/scripts/sysmon/proc_metrics.py
@@ -117,5 +117,7 @@
 
 def _is_sysmon(proc):
   """Return whether proc is a sysmon process."""
-  return (proc.cmdline()[0].endswith('python')
-          and proc.cmdline()[1:3] == ['-m', 'chromite.scripts.sysmon'])
+  cmdline = proc.cmdline()
+  return (cmdline and
+          cmdline[0].endswith('python') and
+          cmdline[1:3] == ['-m', 'chromite.scripts.sysmon'])