Check facets configuration imported when index enabled.

The facets configuration won't be enabled when the index has the
field added if the index is disabled at the time. Check when
the index is updated, if it is enabled and has the field, create
the facet (the create method also checks that it doesn't already
exist).

localgov_directories.module

@@ -193,6 +193,18 @@ function localgov_directories_field_config_delete(FieldConfigInterface $field) {
   }
 }
 
+/**
+ * Implements hook_ENTITY_TYPE_update().
+ */
+function localgov_directories_search_api_index_update(IndexInterface $index) {
+  if ($index->status()
+    && $index->getField(Directory::FACET_INDEXING_FIELD)
+  ) {
+    \Drupal::classResolver(ConfigurationHelper::class)
+      ->createFacet(Directory::FACET_CONFIG_ENTITY_ID, Directory::FACET_CONFIG_FILE);
+  }
+}
+
 /**
  * Implements theme preprocess_facets_item_list().
  *