Update the helper function to cat is not Non

Otherwise, cat = 0 won't be written to the data file

Author: qx0731

introduction_to_amazon_algorithms/deepar_synthetic/deepar_synthetic.ipynb

@@ -265,7 +265,7 @@
    "source": [
     "def series_to_obj(ts, cat=None):\n",
     "    obj = {\"start\": str(ts.index[0]), \"target\": list(ts)}\n",
-    "    if cat:\n",
+    "    if cat is not None:\n",
     "        obj[\"cat\"] = cat\n",
     "    return obj\n",
     "\n",