public static <L, M, R, S> ImmutableQuartet<L, M, R, S> nullTriple() {
        return NULL;
    }
Returns an immutable quartet of nulls.

@param <L> the f1 element of this quartet. Value is {@code null}.
@param <M> the f2 element of this quartet. Value is {@code null}.
@param <R> the f3 element of this quartet. Value is {@code null}.
@param <S> the f4 element of this quartet. Value is {@code null}.
@return an immutable quartet of nulls.
inlong-manager/manager-common/src/main/java/org/apache/inlong/manager/common/tool/excel/tuple/ImmutableQuartet.java