Ensure broadcast with no areas cannot be approved

This is already checked by admin app, but we want to have a
fallback safety measure as a precaution.

app/broadcast_message/rest.py

@@ -59,6 +59,11 @@ def _update_broadcast_message(broadcast_message, new_status, updating_user):
                 f'User {updating_user.id} cannot approve their own broadcast_message {broadcast_message.id}',
                 status_code=400
             )
+        elif not broadcast_message.areas:
+            raise InvalidRequest(
+                f'broadcast_message {broadcast_message.id} has no selected areas and so cannot be broadcasted.',
+                status_code=400
+            )
         else:
             broadcast_message.approved_at = datetime.utcnow()
             broadcast_message.approved_by = updating_user