Add and apply EmptyLineAfterGuardClause rule

We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.

In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.

app/helpers/budget_headings_helper.rb

@@ -7,6 +7,7 @@ module BudgetHeadingsHelper
 
   def heading_link(assigned_heading = nil, budget = nil)
     return nil unless assigned_heading && budget
+
     heading_path = budget_investments_path(budget, heading_id: assigned_heading&.id)
     link_to(assigned_heading.name, heading_path)
   end