When `valuator_group` was `nil`, `[valuator_group&.investment_ids]` is
evaluated to `nil`, and so we were adding an extra element to the array.
We could add a `compact` call to the resulting array, but I find it
easier to convert `nil` to an array using `to_a`.
Eventhough some of us sentimentals still like the syntax `to_not` the current trend is to move to the new syntax `not_to`.
In this commit we are updating the references of expectations that used `to_not` to `not_to`.
