def set_if_empty(*keys, value: nil, &block)
      return unless deep_find(@settings, keys.last.to_s).nil?
      block ? set(*keys, &block) : set(*keys, value: value)
    end
def set_if_empty(*keys, value: nil, &block)
      return unless deep_find(@settings, keys.last.to_s).nil?
      block ? set(*keys, &block) : set(*keys, value: value)
    end
Set a value for a composite key if not present already

 @param [Array[String|Symbol]] keys
   the keys to set value for

 @api public