Lack of explicit nullable types in Cli class triggers deprecation notices in PHP 8.4

These two showed up while I was building out a sample application:

```
Garden\Cli\Cli::parse(): Implicitly marking parameter $argv as nullable is deprecated, the explicit nullable type must be used instead
```

```
Garden\Cli\Cli::validateType(): Implicitly marking parameter $def as nullable is deprecated, the explicit nullable type must be used instead
```

