====== Option +nil_value+

Specifies the object that is to be substituted for each null (no-text) field.

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

With the default, +nil+:
  CSV.parse_line('a,,b,,c') # => ["a", nil, "b", nil, "c"]

With a different object:
  CSV.parse_line('a,,b,,c', nil_value: 0) # => ["a", 0, "b", 0, "c"]
