Do not fill pygal chart with None value

pygal call max on the values so it raises a TypeError when comparing the
number with None on Python3.

issue8147
review50791002

sale_reporting.py

@@ -167,7 +167,7 @@ class Abstract(ModelSQL):
         name = name[:-len('_trend')]
         if pygal:
             chart = pygal.Line()
-            chart.add('', [getattr(ts, name) if ts else None
+            chart.add('', [getattr(ts, name) if ts else 0
                     for ts in self.time_series_all])
             return chart.render_sparktext()