Form API placeholder attribute rendered as "- Select -"
According to the documentation: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21Element%21Select.php/class/Select

#empty_option: (optional) The label to show for the first default option. By default, the label is automatically set to &quot;- Select -&quot; for
a required field and &quot;- None -&quot; for an optional field.