public Kv nullLiteral(String nullLiteral) {
        checkNotNull(nullLiteral);
        internalProperties.putString(TableFormatConstants.FORMAT_NULL_LITERAL, nullLiteral);
        return this;
    }
Sets the null literal string that is interpreted as a null value
(disabled by default).

@param nullLiteral null literal (e.g. "null" or "n/a")
inlong-sort/sort-formats/format-row/format-kv/src/main/java/org/apache/inlong/sort/formats/kv/Kv.java