How to not add empty tasks

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

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

  //each element 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 list item with the text from #new-text
  var listItem = createNewTaskElement(taskInput.value);

  if(taskInput.value) {
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
  } else {
    alert("You must enter a task.");
  }

  //append listItem to incompleteTaskHolder
  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 containsClass = listItem.classList.contains("editMode");
  var editBtn = listItem.getElementsByTagName('button')[0];

    //if the class of the parent is .editMode
  if (containsClass) {
      //switch from editMode
      //label text become the input's value
    label.innerText = editInput.value;
    editBtn.innerText = "Edit";
  } else {
      //switch to .editMode
      //input value becomes the label's text
    editInput.value = label.innerText;
    editBtn.innerText = "Save";
  }  
      //Toggle .editMode on the list item
    listItem.classList.toggle("editMode");
}

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


//mark a task 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 incomplete
var taskIncomplete = function() {
  console.log("task incomplete...");
      //append the task list to the #uncompleted-tasks
  var listItem = this.parentNode;
  incompleteTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskCompleted);

}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
  console.log("Bind list item events");
  //select it'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 complete button
  checkBox.onchange = checkBoxEventHandler;

}

//set the click handler to the addTask function
addButton.onclick = addTask;

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


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