autotest: Initialize tko.models.job.index attribute

BUG=chromium:833988
TEST=None

Change-Id: I63cb1a8530dbfb3be0229ba65ad3b0982cf3e22a
Reviewed-on: https://chromium-review.googlesource.com/1018722
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/tko/db.py b/tko/db.py
index df15372..d604dea 100644
--- a/tko/db.py
+++ b/tko/db.py
@@ -454,9 +454,7 @@
                 'board': job.board,
                 'suite': job.suite}
 
-        # TODO(ntang): check job.index directly.
-        is_update = hasattr(job, 'index')
-        if is_update:
+        if job.index is not None:
             self.update('tko_jobs', data, {'job_idx': job.index}, commit=commit)
         else:
             self.insert('tko_jobs', data, commit=commit)