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

config/locales/en/admin.yml

@@ -1417,6 +1417,7 @@ en:
         level_2_user: Level 2 users
         proposal_created: Citizen proposals
       budgets:
+        no_data_before_balloting_phase: "There isn't any data to show before the balloting phase."
         title: "Participatory Budgets - Participation stats"
         supporting_phase: Supporting phase
         balloting_phase: Final voting