Allow suppress for NullableTypeForNullDefaultValue sniff

[simPod]

Eg. Symfony still supports ancient PHP 7.1 so type hints are often declared as

    /**
     * Gets the listeners of a specific event or all listeners sorted by descending priority.
     *
     * @return array The event listeners for the specified event, or all event listeners by event name
     */
    public function getListeners(string $eventName = null);

with missing ? for nullable types. It is not possible to type hint implementations as

    public function getListeners(?string $eventName = null) {}

Therefore there's need for suppressing the SlevomatCodingStandard.TypeHints.NullableTypeForNullDefaultValue.NullabilitySymbolRequired

[kukulich]

Thanks.