Uninitialized string offset & Undefined array key

[garak]

While testing some cf validation, I stumbled upon these warnings:

Warning: Uninitialized string offset 13 in /myproject/vendor/davidepastore/codice-fiscale/src/AbstractCalculator.php on line 155

Warning: Undefined array key "" in /myproject/vendor/davidepastore/codice-fiscale/src/AbstractCalculator.php on line 156

Warning: Uninitialized string offset 14 in /myproject/vendor/davidepastore/codice-fiscale/src/AbstractCalculator.php on line 155

Warning: Undefined array key "" in /myproject/vendor/davidepastore/codice-fiscale/src/AbstractCalculator.php on line 156

This is similar to issue #36 but IMHO is more serious, because in this case it doesn't depend on any mistaken arguments.

I can propose a PR, which is by the way as simple as:

<             $k = $temporaryCodiceFiscale[$i];
<             $sum = $sum + $dictionaryArray[$k];
---
>             $k = $temporaryCodiceFiscale[$i] ?? '';
>             $sum += $dictionaryArray[$k] ?? 0;