Since I use my framework I don't really care about the CAML, but it should be something like :
<query>
<Query>
<Where>
<IsNull><FieldRef Name="Assign_x0020_Date"></FieldRef></IsNull>
</Where>
</Query>
</query>
BTW you may want to try my framework : http://aymkdn.github.io/SharepointPlus/ The syntax will be (for the same thing) :
function loadPrioritizedList() {
$("#tasksUL").empty();
$SP().list(targetListName).get({
fields:"Priority_x0020_Number,Edit_x0020_Link,Priority,Top_x0020_Item_x003f_,Purpose,Item_x002d_Task_x0020_Order,Mode,Work_x0020_Status,DueDate,Task_x0020_Type,DAK_x0020_Date,DAK_x0020_No,AssignedTo,Money_x0020_Estimate,ItemStatus,Assign_x0020_Date",
where:"Assign_x0020_Date = ''",
orderby:"Priority_x0020_Number DESC",
rowlimit:listrowlimit
}, function (xData) {
var tdHTML="";
for (var i=xData.length; i--;) {
tdHtml += "<tr class='sortable_row' id=" + xData[i].getAttribute("ID") + ">";
tdHtml += "<td style=\"width:60px;\">" + PriorityFormat(xData[i].getAttribute("Priority_x0020_Number")); + "</td>";
tdHtml += '<td style=\"width:49px;\"><a href=\"'+(xData[i].getAttribute("Edit_x0020_Link")).split(", ")[1] + '\">' + (xData[i].getAttribute("Edit_x0020_Link")).split(", ")[1] + '</a></td>';
tdHtml += "<td style=\"width:83px;\">" + xData[i].getAttribute("Priority") + "</td>";
tdHtml += "<td style=\"width:63px;\">" + TopItem(xData[i].getAttribute("Top_x0020_Item_x003f_")) + "</td>";
tdHtml += "<td style=\"width:300px;\">" + StringChk(xData[i].getAttribute("Purpose")) + "</td>";
tdHtml += "<td style=\"width:125px;\">" + StringChk(xData[i].getAttribute("Item_x002d_Task_x0020_Order")) + "</td>";
tdHtml += "<td style=\"width:40px;\">" + StringChk(xData[i].getAttribute("Mode")) + "</td>";
tdHtml += "<td style=\"width:75px;\">" + StringChk(xData[i].getAttribute("Task_x0020_Type")) + "</td>";
tdHtml += "<td style=\"width:150px;\">" + StringChk(xData[i].getAttribute("Work_x0020_Status")) + "</td>";
tdHtml += "<td style=\"width:100px;\">" + FormatDate(xData[i].getAttribute("DueDate")) + "</td>";
tdHtml += "<td style=\"width:100px;\">" + FormatDate(xData[i].getAttribute("DAK_x0020_Date")) + "</td>";
tdHtml += "<td style=\"width:100px;\">" + StringChk(xData[i].getAttribute("DAK_x0020_No")) + "</td>";
tdHtml += "<td style=\"width:300px;\">" + StringChk(xData[i].getAttribute("AssignedTo")) + "</td>";
tdHtml += "<td style=\"width:125px;\">" + xData[i].getAttribute("Money_x0020_Estimate") + "</td>";
tdHtml += "<td style=\"width:75px;\">" + StringChk(xData[i].getAttribute("ItemStatus")) + "</td>";
tdHtml += "<td style=\"width:100px;\">" + FormatDate(xData[i].getAttribute("Assign_x0020_Date")) + "</td>";
tdHtml += "</tr>";
}
$("#tasksUL").append(tdHtml);
}
});