Merge: lib/ini: give to the user `null` on undefined keys

Merge: lib/ini: give to the user `null` on undefined keys

Since the absence of a key is an ini file is an expected thing, I think it is better that the library return `null` instead of aborting.

This simplifies the library (and remove all aborts and cast on null).
This also simplify clients: a single call and use of advanced nullable features of Nit.

Pull-Request: #1672
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>