Extract the load_map logic from the :load_heading callback

Create a new callback for load_map. In this way we simplify a bit the logic of
:load_heading callback and unify the logic of the map loading.

app/controllers/budgets/investments_controller.rb

@@ -20,6 +20,7 @@ module Budgets
 
     before_action :load_ballot, only: [:index, :show]
     before_action :load_heading, only: [:index, :show]
+    before_action :load_map, only: [:index]
     before_action :set_random_seed, only: :index
     before_action :load_categories, only: :index
     before_action :set_default_investment_filter, only: :index
@@ -134,10 +135,8 @@ module Budgets
         if params[:heading_id].present?
           @heading = @budget.headings.find_by_slug_or_id! params[:heading_id]
           @assigned_heading = @ballot&.heading_for_group(@heading.group)
-          load_map
         elsif @budget.single_heading?
           @heading = @budget.headings.first
-          load_map
         end
       end
 
@@ -180,7 +179,7 @@ module Budgets
       end
 
       def load_map
-        @map_location = MapLocation.load_from_heading(@heading)
+        @map_location = MapLocation.load_from_heading(@heading) if @heading.present?
       end
   end
 end