Handle areas which don’t have population data

If an area has a `count_of_phones` value of `0` it means we don’t have
data about the population.

This means we can’t do the maths to work out the estimated bleed. So we
should return the default amount of bleed of 1,500m instead, which is
something in between what we’d expect for a built up area and a rural
area.

app/broadcast_areas/__init__.py

@@ -74,12 +74,14 @@ class BroadcastArea(SortableMixin):
         # https://www.pivotaltracker.com/story/show/174837293
         return self._count_of_phones or 0
 
-    @property
+    @cached_property
     def phone_density(self):
         return self.count_of_phones / self.polygons.estimated_area
 
     @property
     def estimated_bleed_in_m(self):
+        if self.phone_density < 1:
+            return Polygons.approx_bleed_in_degrees * Polygons.approx_metres_to_degree
         estimated_bleed = 5_900 - (math.log(self.phone_density, 10) * 1_250)
         return max(500, min(estimated_bleed, 5000))