Don't allow negativ str_repeats

[mpociot]

As soon as you manually add a newLine output in combination with Prompts, the command throws an exception.

For example, this example command currently breaks with Prompts.

<?php

namespace App\Console\Commands;

use Illuminate\Console\Command;
use function Laravel\Prompts\note;

class TestCommand extends Command
{
    protected $signature = 'app:test-command';

    public function handle()
    {
        note("1");
        note("2");
        $this->output->newLine();
        note("3");
    }
}

In this case, the calculated value for str_repeat becomes negative, which is not allowed as per the PHP docs.

This PR fixes this by only using the values 0 or greater.