Empty headings throw error : Undefined property: stdClass::$content

[kristiana-s]

Hi,
Spotted an issue where an empty heading node would throw an error for no content. This was for entries with linked content , and the content had empty headings.
Undefined property: stdClass::$content

fix was to add an extra check on line 22 of src/Nodes/Heading.php to check that content node exists

if (property_exists($node, 'content') && property_exists($node->content[0], 'text') && !is_null($node->content[0]->text)) {
     $HTMLAttributes['id'] = Str::slug($node->content[0]->text);
}

Happy to create a PR if needed, Thanks for this addon!