Fix #312 PHP type missing when param is `null`

Examples:

    /**
     * [fname description]
     * @param  array      $a [description]
     * @param  array|null $b [description]
     * @return [type]        [description]
     */
    function fname(array $a, array $b = null) {}

    /**
     * [fname description]
     * @param  NS\ClassA      $a [description]
     * @param  NS\ClassB|null $b [description]
     * @return [type]            [description]
     */
    function fname(NS\ClassA $a, NS\ClassB $b = null) {}

    /**
     * [fname description]
     * @param  \NS\ClassA      $a [description]
     * @param  \NS\ClassB|null $b [description]
     * @return [type]             [description]
     */
    function fname(\NS\ClassA $a, \NS\ClassB $b = null) {}