Avoid error when accessing final votes stats before balloting phase

When accessing the URL `/admin/stats/budget_balloting?budget_id=X'
for a budget in a phase prior to the balloting phase, the following
error was raised due to the stats where not calculated yet.
Instead, we'll now show a flash message.

NoMethodError:
  undefined method `[]' for nil:NilClass
  ./app/controllers/admin/stats_controller.rb:82

app/models/abilities/administrator.rb

@@ -66,6 +66,8 @@ module Abilities
       can [:valuate, :comment_valuation], Budget::Investment
       can :create, Budget::ValuatorAssignment
 
+      can(:read_admin_stats, Budget) { |budget| budget.balloting_or_later? }
+
       can [:search, :edit, :update, :create, :index, :destroy], Banner
 
       can [:index, :create, :edit, :update, :destroy], Geozone