title: Remove "Choose an option" from variable product dropdowns in Woocommerce 3
body: <p>I would like to remove from the dropdown of variations in WooCommerce product page following "option":
  <a href="https://i.stack.imgur.com/qSDbn.png" rel="noreferrer"><img src="https://i.stack.imgur.com/qSDbn.png" alt="WooCommerce &quot;Select an option&quot; issue"></a></p>
  
  <p>I found plenty of, apparently not working codes which should do the job. Probably outdated to the latest WooCommerce version.</p>
  
  <p>What I tried and is partially working:</p>
  
  <pre><code>add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'my_wc_filter_dropdown_args', 10 );
  function my_wc_filter_dropdown_args( $args ) {
      $args['show_option_none'] = '';
      return $args;
  }
  </code></pre>
  
  <p>This is only working when I set some text between '', not empty. When it's added into functions.php exactly like above, it's without change and set to default text - "Select an option" like on the picture. I am not sure what's wrong here. I also tried "false" or "none" but didn't work with either option.</p>
  
  <p>If anyone could help me with this I would be grateful.</p>
  
  <p>I am using latest WP 4.9.6 and latest of WooCommerce (whatever version it is). Everything is updated to the latest version, even PHP (7.2).</p>