agregar opción "en blanco"

app/views/posts/template_field/_select.haml

@@ -1,7 +1,7 @@
 - value  = [] if value.nil?
 - values = template.values.empty? ? [value].flatten : template.values
 = select_tag field_name_for_post_as_string(name),
-  options_for_select(values, value),
+  options_for_select([t('posts.blank'), nil] + values, value),
   { class: 'form-control select2',
   multiple: template.multiple?,
   required: template.required?,

config/locales/en.yml

@@ -243,3 +243,4 @@ en:
     url: 'The address must start with http:// or https://'
     invalid_help: It looks like the form is incomplete.  Check the red-colored fields to complete it.
     sending_help: Saving, please wait...
+    blank: Nothing

config/locales/es.yml

@@ -250,3 +250,4 @@ es:
     url: 'La dirección debe comenzar con http:// o https://'
     invalid_help: Parece que el formulario no está completo.  Verifica los campos marcados en rojo para completarlo.
     sending_help: Guardando, por favor espera...
+    blank: En blanco