Update timeseries-queries-and-visualization.ipynb

Added:

"WHERE attributes IS NOT NULL" to make sure that queries actually shows the unnesting. 

to the example query in the "Querying with Flattening / Unnesting" section.

topic/timeseries/timeseries-queries-and-visualization.ipynb

@@ -1492,6 +1492,7 @@
     "       attributes['weather_condition_code'] AS weather_condition_code,\n",
     "       attributes['sensor_information']['software_version'] AS software_version\n",
     "FROM weather_data\n",
+    "WHERE attributes IS NOT NULL\n",
     "ORDER BY location, timestamp\n",
     "```\n",
     "\n",