Return the persisted line in add_investment

This method was returning a boolean value and caused a
`Naming/PredicateMethod` when upgrading rubocop.

So, instead, we're returning the created line when it was successfully
created, and `nil` when it wasn't.

Having said that, I'm not sure why we added the `.persisted?` back in
commit 3eb22ab7b since as far as I can tell we don't use the return
value for anything. The test added in commit da43e9e2e for this change
passes if we simply return `lines.create(investment: investment)`.

For now I'm leaving the `persisted?` check just in case, but removing it
might be fine.

app/models/budget/ballot.rb

@@ -10,7 +10,9 @@ class Budget
     has_many :headings, -> { distinct }, through: :groups
 
     def add_investment(investment)
-      lines.create(investment: investment).persisted?
+      line = lines.create(investment: investment)
+
+      line if line.persisted?
     end
 
     def total_amount_spent