added null value in doc block for $port
added null value in doc block for $port

as null is accepted for an unset-standard port.
somjit2514/basic,akrabat/Slim,opengeek/Slim,JoeBengalen/Slim,jaapverloop/Slim,samsonasik/Slim,RealSelf/Slim,feryardiant/slim,slimphp/Slim,juliangut/Slim,iinux/Slim,Sam-Burns/Slim,dopesong/Slim,AndrewCarterUK/Slim,mnapoli/Slim,iinux/Slim,designermonkey/Slim,foxyantho/Slim,iinux/Slim