Check if a parameter is NULL

Hi,

I am setting the default value of a parameter in a transformation as 'null'. And in the modified javascript of that transformation I want to check if the parameter value is null or not. Simple if condition is not working, i.e
var today = new Date();
var current_hr=today.getHours();

var dd = today.getDate();
var mm = today.getMonth()+1; //January is 0!

var yyyy = today.getFullYear();
if(dd<10){dd='0'+dd}
if(mm<10){mm='0'+mm}
var today_date =yyyy+'-'+mm+'-'+dd;
var rep_hour="";
var man_date_val=getVariable("man_date","");

if (man_date_val==null)
{
var rep_hour=(current_hr*1)-1;
if(rep_hour <=9)
{
hr="0"+rep_hour;
}
else
{
hr=rep_hour;
}
var date_time=today_date+ " " +hr +"%"
var key_time=today_date+ " " +hr +":00:00";
}
else
{
date_time=man_date_val;
var key_time=date_time;
}

Here, even if man_date default value is null(defined in settings of the transformation), it is always going to the else condition. Any suggestion or help???

Thanks in advance.
Poulomi