It should just be:

var liHtml5= "<li>" + $(this).attr("ows_Important_x0020_Link").split(", ")[0] + "</li>";

You have extra parans in there.

It should just be:

var liHtml5= "<li>" + $(this).attr("ows_Important_x0020_Link").split(", ")[0] + "</li>";

You have extra parans in there.

If the column isn't required then you need to adapt your code:

var liHtml5 = "";
var link = $(this).attr("ows_Important_x0020_Link");
if (link.length){
 liHtml5= "<li>" + link.split(", ")[0] + "</li>";
 $("#tasksUL").append(liHtml5);
}