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/controllers/admin/stats_controller.rb

@@ -74,6 +74,9 @@ class Admin::StatsController < Admin::BaseController
 
   def budget_balloting
     @budget = Budget.find(params[:budget_id])
+
+    authorize! :read_admin_stats, @budget, message: t("admin.stats.budgets.no_data_before_balloting_phase")
+
     @user_count = @budget.ballots.select {|ballot| ballot.lines.any? }.count
 
     @vote_count = @budget.lines.count