NullableGetterAssembler should generate the proper @return documentation.

[drupol]

When a method has a nullable return type, the phpdoc block generated is:

    /**
     * @return string
     */
    public function getMessage(): ?string
    {
        return $this->message;
    }

It should be like this:

    /**
     * @return string|null
     */
    public function getMessage(): ?string
    {
        return $this->message;
    }