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

spec/features/admin/stats_spec.rb

@@ -189,6 +189,21 @@ describe "Stats" do
           expect(page).to have_content 0
         end
       end
+
+      scenario "hide final voting link" do
+        visit admin_stats_path
+        click_link "Participatory Budgets"
+
+        within("#budget_#{@budget.id}") do
+          expect(page).not_to have_link "Final voting"
+        end
+      end
+
+      scenario "show message when accessing final voting stats" do
+        visit budget_balloting_admin_stats_path(budget_id: @budget.id)
+
+        expect(page).to have_content "There isn't any data to show before the balloting phase."
+      end
     end
 
     context "Balloting phase" do