Markdown preview may return error when the field is empty

[TimeZHero]

• Laravel Version: 9.31.0
• Nova Version: 4.15.1
• PHP Version: 8.1
• Database Driver & Version: MySQL 8
• Operating System and Version: Ubuntu 22.04
• Browser type and version: Chrome, latest
• Reproduction Repository: None

Description:

By clicking "Preview" on Markdown field (currently using default driver) when the field is empty, I receive an error stating that the value is expected to be string rather than null:

As the error states, the issue happens in Laravel\Nova\Fields\Markdown\DefaultPreset:15

<?php

namespace Laravel\Nova\Fields\Markdown;

use Illuminate\Support\Str;

class DefaultPreset implements MarkdownPreset
{
    /**
     * Convert the given content from markdown to HTML.
     *
     * @param  string  $content
     * @return string
     */
    public function convert(string $content) // if we change it to ?string it would just break later on
    {
        return Str::markdown($content);
    }
}

Obviously enough, this issue is shared among all the drivers, hence the optimal fix should be placed in Laravel\Nova\Fields\Markdown:105

/**
 * Return a preview for the given field value.
 *
 * @param  string  $value
 * @return string
 */
public function previewFor($value)
{
    // could add __( ) for allowing translations, default it to empty string or expose a method to set the default value
    return $this->renderer()->convert($value ?? 'Nothing to preview');
}

Detailed steps to reproduce the issue on a fresh Nova installation:

1. Add a Markdown field inside any resource
2. No specific driver is required
3. Go to edit view and try to Preview the markdown while it is empty

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