title: Cancel out a prompt if something equals something
body: <p>I got this prompt that makes it change the document.title when the prompt has been submitted, but i want it to make it so that if my prompt equals something, it cancels out the prompt, how to do that?
  here is my HTML and JS, also it changes the h1 text, in the snippet it wont change the stackoverflow title cause it's already pre defined.</p>
  <p>P.S: i know it's gonna be something small i missed, but pardon me for that i'm still a beginner &gt;_&lt;</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>const h1 = document.getElementById('h1');
  function myFunction() {
      let mainTitle = "Enter website title..."
      let websiteTitle = prompt("What is your website title?", mainTitle)
      if(websiteTitle != null ) {
          document.title = websiteTitle;
          h1.innerText = websiteTitle;
      } else if(document.title === mainTitle) {
          websiteTitle = null;
      }
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;!DOCTYPE html&gt;
  &lt;html lang="en"&gt;
  &lt;head&gt;
      &lt;meta charset="UTF-8"&gt;
      &lt;meta name="viewport" content="width=device-width, initial-scale=1.0"&gt;
      &lt;title&gt;&lt;/title&gt;
      &lt;link rel="stylesheet" href="pets.css"&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;h1 id="h1"&gt;Website Title here!&lt;/h1&gt;
  &lt;button onclick="myFunction()"&gt;Change Website Title&lt;/button&gt;
  &lt;script src="pets.js"&gt;&lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>