Handle areas with missing data

At the moment there are some areas which have:
- a `count_of_phones` value of `None`
- no sub-areas

This is wrong, but until we fix the data the phone counting code needs
to handle this.

This commit:
- adds the `or 0` in the right place (where it will catch these areas
  with missing data)
- adds a test which checks these areas, and compares them to other kinds
  of areas

app/models/broadcast_message.py

@@ -124,7 +124,7 @@ class BroadcastMessage(JSONModel):
     @property
     def count_of_phones(self):
         return round_to_significant_figures(
-            sum(area.count_of_phones or 0 for area in self.areas),
+            sum(area.count_of_phones for area in self.areas),
             2
         )