Specifies string used as a NULL value representer.
property NullValue: string;
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.