Copy the not.null hints from teaching to generalise
Copy the not.null hints from teaching to generalise

Using `null` instead of testing equality against `[]` has a real advantage: it lets you drop the `Eq` constraint on the contents, and operate on any `Foldable` rather than just on lists. As such, this is a generalisation useful in real programs, and not just for teaching.

ndmitchell/hlint,ndmitchell/hlint