Refactor code for empty terms hidding.

Closes #2884294

config/schema/taxonomy_menu.schema.yml

@@ -29,3 +29,6 @@ taxonomy_menu.taxonomy_menu.*:
     use_term_weight_order:
       type: boolean
       label: 'Use term weight order'
+    hide_empty_terms:
+      type: boolean
+      label: 'Hide empty terms'