Display message in budget's index when there are no budgets

When there are no budgets we were seeing an exception in the budgets’
index

There are two parts to take into account here:
1) Making sure there is a current_budget present, otherwise we display
the “no budgets” message

2) The map helper is called from the controller, so we need to make
sure current_budget is present there too

Note: We could have added a bunch of `try` statements in the budgets’s
index, instead of using a conditional, however there are quite a few
`current_budget` calls so it seems more appropriate to use a conditional

spec/features/budgets/budgets_spec.rb

@@ -117,6 +117,13 @@ feature 'Budgets' do
       end
     end
 
+    scenario "No budgets" do
+      Budget.destroy_all
+
+      visit budgets_path
+
+      expect(page).to have_content "There are no budgets"
+    end
   end
 
   scenario 'Index shows only published phases' do