====== Option +empty_value+

Specifies the object that is to be substituted
for each field that has an empty \String.

Default value:
  CSV::DEFAULT_OPTIONS.fetch(:empty_value) # => "" (empty string)

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

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