Need help with Form Validation Rule: "required_without"

I'm using Laravel 7 to create a forum board system, in which a Board model has an inverse ManyToOne relationship to a Category model, and optionally to itself as "parent board".

In my front-end form I have these select elements:

<select name="category_id" id="category_id">
   <option value="" selected>Select a Category</option>
   @foreach($categories as $category)
   <option value="{{ $category->id }}">{{ $category->name}}</option>
   @endforeach
</select>

<select name="parent_id" id="parent_id">
   <option value="" selected>Optional Parent Board</option>
   @foreach($boards as $board)
   <option value="{{ $board->id }}">{{ $board->name}}</option>
   @endforeach
</select>

On the backend I'm using a FormRequest to validate the form so that it only requires category_id if parent_id is not present, and vice-versa. This is so that if the user creates/edits a board, and assigns a parent to it, the category_id becomes the same as the parent's; but if it doesn't have a parent, then it must have a category_id.

I'm using the following rules for that:

public function rules()
{
   return [
      'category_id' => 'required_without:parent_id|integer',
      'parent_id' => 'required_without:category_id|integer',
   ];
}

The problem I'm having right now is that upon form submission, the integer rule triggers if either of the fields are null, and returns the error "The category/parent id must be an integer". The only answers I've found are to enable ConvertEmptyStringsToNul middleware, but it's already active in my app, as I can see the value null (not the string) when dumping the request.

How can I fix this validation?