매뉴얼 내용입니다.

The NULL portion of the column-spec indicates how to treat certain input
values as NULL values when loading into the table column. These characters
can include BLANKS, ZEROS, or any other list of literals you define. When
specifying a NULL value or reading a NULL value from the source file, the
destination column must be able to contain NULLs.
ZEROS are interpreted as follows: the cell is set to NULL if (and only if) the
input data (before conversion, if ASCII) is all binary zeros (and not character
zeros).
• If the input data is character zero, then:
a NULL (ZEROS) never causes the cell to be NULL.
b NULL ('0') causes the cell to be NULL.
• If the input data is binary zero (all bits clear), then:
a NULL (ZEROS) causes the cell to be NULL.
b NULL ('0') never causes the cell to be NULL.

For example, if your LOAD statement includes col1 date('yymmdd')
null(zeros) and the date is 000000, you receive an error indicating that
000000 cannot be converted to a DATE(4). To get load to insert a NULL value
in col1 when the data is 000000, write the NULL clause as null('000000'),
or modify the data to equal binary zeros and use NULL(ZEROS).
If the length of a VARCHAR cell is zero and the cell is not NULL, you get a
zero-length cell. For all other data types, if the length of the cell is zero, Sybase
IQ inserts a NULL. This is ANSI behavior. For non-ANSI treatment of zerolength
character data, set the Non_Ansi_Null_Varchar database option.