Disabled elements do not get submitted with forms (hence the null value), so the easiest way to get around this would be to use a hidden element.
Something like:
if('${actionBean.list1.size()}' > 0){
var dropDown = $('#dropdownA');
$("myHiddenElement").val(dropDown.val());
dropDown.prop("disabled", true);
}
Then you can check if dropDown has a value on the server and if not, default to myHiddenElement.