How to not assign an ID when creating a record in the database?

Question

Laravel

When creating an entry (the user creates a password, selects a category). How to implement so that you can create a record without a category_id, check the general box and in the database this record becomes without a category_id.

Creation method in controller:

public function store(Request $request) {
        $id = Auth::user()->id;
        $pass = new Pass();
        $pass->title = $request->title;
        $pass->category_id = $request->category_id;
        $pass->user_id = $id;
        $pass->save();
        return redirect()->route('home');
    }

A fragment from the view in which we create an entry (password) and select a category

<div class="form-group">
    <input name="title" type="text" class="form-control"  required value="{{ $pass->title ?? ''}}">
</div>
<div class="form-group">
    <label for="category_id">Категория</label>
    <select name="category_id" id="category_id" class="form-control">
        @foreach ($categorys as $category)
            <option value="{{$category->id}}">{{$category->title}}</option>
        @endforeach
    </select>
</div>