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.
413d0ed9be