Fix non-nullable internal function deprecation notice
Fix non-nullable internal function deprecation notice

In PHP 8.1, [Passing `null` to a non-nullable parameter causes a deprecation notice](https://php.watch/versions/8.1/internal-func-non-nullable-null-deprecation).

In Psysh, `Psy\VarDumper\Dumper::style()` method has a `preg_split` call that triggers this deprecation:

```
Deprecated: preg_split(): Passing null to parameter #3 ($limit) of type int is deprecated in src/VarDumper/Dumper.php on line 77
```

This PR fixes it by changing the `$limit` parameter value to the default `-1` from `null`.

bobthecow/psysh,bobthecow/psysh