49 - add explicit nullable declarations

[mattgorle]

This should address #49 by adding the following explicit nullable declarations:

1. Cli::parse: $argv
2. Cli::validateType $def

The underlying issue is not visible when testing with PHP 8.4 and PHPUnit 8, so there is no clear difference when running the test suite.

However, when testing from an external project with PHP 8.4 and PHPUnit 12, two deprecation notices appear before the change and disappear after the change.