string or null を返すメソッドで型宣言できない

gistfile1.php

<?php
// PHP7.1以上だったらこれでOK
class UserPHP71 {
	/**
	 * @var string
	 */
	private $name;
	/**
	 * @var string|null
	 */
	private $otherName;

	public function __construct(string $name, string $otherName = null) { // string $otherName = null だとstringもnullもわたせる
		$this->name = $name;
		$this->otherName = $otherName;
	}

	/**
	 * @return string
	 */
	public function getName() : string {
		return $this->name;
	}

	/**
	 * @return string|null
	 */
	public function getOtherName() : ?string {
		return $this->otherName;
	}
}
$user = newUserPHP71('Ryuji');
echo $user->getOtherName(); // echo nullになる


class User {
	/**
	 * @var string
	 */
	private $name;
	/**
	 * @var string|null
	 */
	private $otherName;

	public function __construct(string $name, string $otherName = null) { // string $otherName = null だとstringもnullもわせる
		$this->name = $name;
		$this->otherName = $otherName;
	}

	/**
	 * @return string
	 */
	public function getName() : string {
		return $this->name;
	}

	/**
	 * @return string|null
	 */
	public function getOtherName() : string {
		return $this->otherName;
	}
}

$user = new User('Ryuji');
echo $user->getOtherName(); // <- Fatal error: Uncaught TypeError: Return value of User::getOtherName() must be of the type string, null returned

class User改 {
	/**
	 * @var string
	 */
	private $name;
	/**
	 * @var string|null
	 */
	private $otherName;

	public function __construct(string $name, string $otherName = null) {
		$this->name = $name;
		$this->otherName = $otherName;
	}

	/**
	 * @return string
	 */
	public function getName() : string {
		return $this->name;
	}

	/**
	 * @return string|null
	 */
	public function getOtherName() { //<- 戻り値型を指定できないのがきになる
		return $this->otherName;
	}
}

https://twitter.com/RyujiAMANO/status/1113655976756834304
@suin さんに教えてもらった。 ?string でstring|null