Add #empty_option for required fields

Closes #3385192

src/Plugin/Field/FieldWidget/PluginReferenceSelectWidget.php

@@ -109,6 +109,10 @@ class PluginReferenceSelectWidget extends WidgetBase {
       '#options' => $this->getOptions($items->getEntity()),
     ] + $element;
 
+    if ($items->getFieldDefinition()->isRequired()) {
+      $element['plugin_id']['#empty_option'] = $this->t('- Select -');
+    }
+
     $element = $this->singleConfigurationFormElement($items, $delta, $element, $form, $form_state);
 
     return $element;