Specifies string used as a NULL value representer.
If coNullValue not included in Options or empty string specified then default values used. The default is \N (backslash-N) in text mode, and a empty value with no quotes in CSV mode. You might prefer an empty string even in text mode for cases where you don't want to distinguish NULLs from empty strings.
When using LoadFromXXX methods, any data item that matches this string will be stored as a NULL value, so you should make sure that you use the same string as you used with SaveToXXX methods.