title: Update custom user meta value using checkbox
body: <p>I am using <em>"<a href="https://stackoverflow.com/questions/56072021/integrate-sendgrid-newsletter-subscription-form-on-woocommrece-register-page/56072958#56072958">Integrate Sendgrid Newsletter Subscription form on WooCommrece Register Page</a>"</em> answer code to one of my previous answers. On Woocommerce account details section of My account page, I have a custom user meta field which is <code>receive_newsletter</code> which by default has a value of "1" when is checked, like:</p>
  
  <p><a href="https://i.stack.imgur.com/op3SL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/op3SL.png" alt="enter image description here"></a></p>
  
  <p>When it is unchecked the value will be empty and make it unchecked. </p>
  
  <p>However when I save the changes it will not update. </p>
  
  <p>This would be supposedly an output:</p>
  
  <p><a href="https://i.stack.imgur.com/j9WY3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/j9WY3.png" alt="enter image description here"></a></p>