I've tried to create a model like yours and set up same constraints, and creating models with empty or null parent_id works just fine.

Could you post your migration file?

Mine is like this:

	public function up()
	{
		Schema::create('categories', function(Blueprint $table)
		{
			$table->increments('id');
			$table->integer('parent_id')->unsigned()->nullable();
			$table->string('name');

			$table->foreign('parent_id')->references('id')->on('categories')
			                            ->onDelete('set null')
			                            ->onUpdate('cascade');
		});
	}

I have the same migration. You can see it in this gist: https://gist.github.com/Stichoza/444b16aecc26b73b9ede

Do you get the same error when trying this in php artisan tinker:

\App\Category::create(['parent_id' => null, 'title' => 'foobar']);