====== Option +write_nil_value+

Specifies the object that is to be substituted for each +nil+ field.

Default value:
  CSV::DEFAULT_OPTIONS.fetch(:write_nil_value) # => nil

Without the option:
  str = CSV.generate_line(['a', nil, 'c', nil])
  str # => "a,,c,\n"

With the option:
  str = CSV.generate_line(['a', nil, 'c', nil], write_nil_value: "x")
  str # => "a,x,c,x\n"
