In Vue v2, How do I verify if a value entered into a form is empty?

I've tried many different ways to do this with both alert() and simple if-else statements, and even what is at this link: https://v2.vuejs.org/v2/cookbook/form-validation.html

But nothing works! What am I doing wrong?

What is in my index.html:

  <div id="app">
    <div class = "addTask">
    <h1>A List of Tasks</h1>
    <p v-show="activeItems.length === 0">You are done with all your tasks! Celebrate!</p>
      <form @submit.prevent="addItem">
        <input type="text" v-model="title">
        <button type="submit">+</button>
      </form>
    </div>

This is what I have in scripts.js:

var app = new Vue({
  el: '#app',
  data () {
    return {
      // errors: [],
      items: [{
        userId: 0,
        id: 0,
        title: "",
        completed: false,
        }],
        title: '',
        show: 'all',
    }
  },
  // Using axios to asynchrously query the API
  mounted () {
    axios
      .get("https://jsonplaceholder.typicode.com/todos")
      .then(response => this.items = response.data)
  },
  computed: {
    activeItems() {
      this.saveItems;
      return this.items.filter(item => {
        return !item.completed;
      });
    },
    filteredItems() {
      // An active state denotes the task is not yet completed
      if (this.show === 'active')
        return this.items.filter(item => {
          return !item.completed;
        });
      if (this.show === 'completed')
        return this.items.filter(item => {
          return item.completed;
        });
        //  This makes it so added tasks go to the top, not bottom
      return this.items.reverse();
    },
    
  },
  methods: {
    addItem() {

      if(this.items != 0) {
        this.items.push({
          title: this.title,
          completed: false
        })
        this.title = "";
      }
      else {
        alert("Please enter a task.")
      }