How can I prevent the "Add" button from creating a task without an input value?

//Problem: User interaction doesn't provide desired results.
//Solution: Add interactivity so the user can manage daily tasks.

//Add a new task
var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementsByTagName("button")[0]; //first-button
var incompleteTasksHolder = document.getElementById("incomplete-tasks"); //incomplete-tasks
var completedTasksHolder = document.getElementById("completed-tasks"); //completed-tasks

//New Task List Item
var createNewTaskElement = function (taskString) {
    //Create List Item
    var listItem = document.createElement("li");

        //input (checkbox)
        var checkBox = document.createElement("input"); //checkbox
        //label
        var label = document.createElement("label");
        //input (text)
        var editInput = document.createElement("input"); //text
        //button.edit
        var editButton = document.createElement("button");
        //button.delete
        var deleteButton = document.createElement("button");

    //Each elements needs modifying
    checkBox.type = "checkbox";
    editInput.type = "text";
    editButton.innerText = "Edit";
    editButton.className = "edit";
    deleteButton.innerText = "Delete";
    deleteButton.className = "delete";

    label.innerText = taskString;

    //Each element needs appending
    listItem.appendChild(checkBox);
    listItem.appendChild(label);
    listItem.appendChild(editInput);
    listItem.appendChild(editButton);
    listItem.appendChild(deleteButton);

    return listItem;
};

//Add a new task
var addTask = function () {
    console.log("Add task...");
    //Create a new list item with the text from #new-task:
    var listItem = createNewTaskElement(taskInput.value);
    //Append listItem to incompleteTasksHolder
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);

    taskInput.value = "";   
    };

//Edit an existing task
var editTask = function () {
    console.log("Edit task...");

    var listItem = this.parentNode;
    var editInput = listItem.querySelector("input[type=text]");
    var label = listItem.querySelector("label");
    var editButton = listItem.getElementsByTagName('button')[0];

    var containsCLass = listItem.classList.contains("editMode");
        //if the class of the parent is .editMode
        if (containsCLass) {
            //Switch from .editMode
            //label text become the input's value
            label.innerText = editInput.value;
            editButton.innerText = "Edit";
        } else {
            //Switch to .editMode
            //input value becomes the label's text
            editInput.value = label.innerText;
            editButton.innerText = "Save";
        }

        //Toggle .editMode on the parent
        listItem.classList.toggle("editMode");
    };

//Delete an existing task
var deleteTask = function () {
    console.log("Delete task...");
    var listItem = this.parentNode;
    var ul = listItem.parentNode;

    //Remove the parent list item from the ul
    ul.removeChild(listItem);
};

//Mark a task as complete
var taskCompleted = function () {
    console.log("Task complete...");
    //Append the task list item to the #completed-tasks 
    var listItem = this.parentNode;
    completedTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskIncomplete);
};

//Mark a task as incomplete
var taskIncomplete = function () {
    console.log("Task incomplete...");
    //Append the task list item to the #incompleted-tasks
    var listItem = this.parentNode;
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
};

var bindTaskEvents = function (taskListItem, checkBoxEventHandler) {
    console.log("Bind list item events");
    //select taskListItem's children
    var checkBox = taskListItem.querySelector("input[type=checkbox]");
    var editButton = taskListItem.querySelector("button.edit");
    var deleteButton = taskListItem.querySelector("button.delete");
        //bind editTask to edit button
        editButton.onclick = editTask;
        //bind deleteTask to delete button
        deleteButton.onclick = deleteTask;
        //bind checkBoxEventHandler to checkbox 
        checkBox.onchange = checkBoxEventHandler;   
};

var ajaxRequest = function() {
    console.log("AJAX request");
};

//Set the click handler to the addTask function
// addButton.onclick = addTask;
addButton.addEventListener("click", addTask);
addButton.addEventListener("click", ajaxRequest);

//Cycle over incompleteTasksHolder ul list items
for (var i = 0; i < incompleteTasksHolder.children.length; i++) {
    //bind events to list item's children (taskCompleted)
    bindTaskEvents(incompleteTasksHolder.children[i],taskCompleted);
}

//Cycle over completedTasksHolder ul list items
for (var i = 0; i < completedTasksHolder.children.length; i++) {
    //bind events to list item's children (taskIncomplete)
    bindTaskEvents(completedTasksHolder.children[i],taskIncomplete);
}