KryptonInputBox should return null instead of "" when user clicks the "X" button on the top right

[xiaoyuvax]

KryptonInputBox should return null instead of "" when user clicks the "X"(close or cancel) button on the top right, so that user's intention of inputing empty string can be distiguished from clicking the "X" button

var input = KryptonInputBox.Show("prompt", "title", "defaultvalue");
if (input==null)
{
//user pressed the "X" button 
}
else if (input =="")
{
//user cleared the inputarea.
}