CSV::FieldsConverter#convert

# File tmp/rubies/ruby-3.0.5/lib/csv/fields_converter.rb, line 47
def convert(fields, headers, lineno)
  return fields unless need_convert?

  fields.collect.with_index do |field, index|
    if field.nil?
      field = @nil_value
    elsif field.is_a?(String) and field.empty?
      field = @empty_value unless @empty_value_is_empty_string
    end
    @converters.each do |converter|
      break if field.nil? and @accept_nil
      if converter.arity == 1  # straight field converter
        field = converter[field]
      else                     # FieldInfo converter
        if headers
          header = headers[index]
        else
          header = nil
        end
        field = converter[field, FieldInfo.new(index, lineno, header)]
      end
      break unless field.is_a?(String)  # short-circuit pipeline for speed
    end
    field  # final state of each field, converted or original
  end
end