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.

spec/models/poll/ballot_spec.rb

@@ -53,43 +53,43 @@ describe Poll::Ballot do
     describe "Money" do
       it "is not valid if insufficient funds" do
         investment.update!(price: heading.price + 1)
-        expect(poll_ballot.add_investment(investment.id)).to be false
+        expect(poll_ballot.add_investment(investment.id)).to be nil
       end
 
       it "is valid if sufficient funds" do
         investment.update!(price: heading.price - 1)
-        expect(poll_ballot.add_investment(investment.id)).to be true
+        expect(poll_ballot.add_investment(investment.id)).to be_truthy
       end
     end
 
     describe "Heading" do
       it "is not valid if investment heading is not valid" do
-        expect(poll_ballot.add_investment(investment.id)).to be true
+        expect(poll_ballot.add_investment(investment.id)).to be_truthy
 
         other_heading = create(:budget_heading, group: group, price: 10000000)
         other_investment = create(:budget_investment, :selected, price: 1000000, heading: other_heading)
 
-        expect(poll_ballot.add_investment(other_investment.id)).to be false
+        expect(poll_ballot.add_investment(other_investment.id)).to be nil
       end
 
       it "is valid if investment heading is valid" do
-        expect(poll_ballot.add_investment(investment.id)).to be true
+        expect(poll_ballot.add_investment(investment.id)).to be_truthy
 
         other_investment = create(:budget_investment, :selected, price: 1000000, heading: heading)
 
-        expect(poll_ballot.add_investment(other_investment.id)).to be true
+        expect(poll_ballot.add_investment(other_investment.id)).to be_truthy
       end
     end
 
     describe "Selectibility" do
       it "is not valid if investment is unselected" do
         investment.update!(selected: false)
-        expect(poll_ballot.add_investment(investment.id)).to be false
+        expect(poll_ballot.add_investment(investment.id)).to be nil
       end
 
       it "is valid if investment is selected" do
         investment.update!(selected: true, price: 20000)
-        expect(poll_ballot.add_investment(investment.id)).to be true
+        expect(poll_ballot.add_investment(investment.id)).to be_truthy
       end
     end
 
@@ -101,7 +101,7 @@ describe Poll::Ballot do
       end
 
       it "is valid if investment belongs to the poll's budget" do
-        expect(poll_ballot.add_investment(investment.id)).to be true
+        expect(poll_ballot.add_investment(investment.id)).to be_truthy
       end
     end
 
@@ -112,7 +112,7 @@ describe Poll::Ballot do
       end
 
       it "is valid if does not already exist" do
-        expect(poll_ballot.add_investment(investment.id)).to be true
+        expect(poll_ballot.add_investment(investment.id)).to be_truthy
       end
     end
   end