DECLARE @delimiter NVARCHAR(MAX) = '||'
DECLARE @null_replacement NVARCHAR(MAX) = '^^'

;
WITH source_data AS
(
SELECT 0 AS hk,'MA1' AS ma_attribute, null col_a, null col_b
UNION all
SELECT 0 AS hk,'MA2' AS ma_attribute, NULL col_a, NULL col_b
UNION all
SELECT 0 AS hk,'MA2' AS ma_attribute, NULL col_a, NULL col_b
UNION all
SELECT 0 AS hk,'MA3' AS ma_attribute, null col_a, null col_b
UNION all
SELECT 1 AS hk,'MA2' AS ma_attribute, 'col_a' col_a, NULL col_b
UNION all
SELECT 1 AS hk,'MA3' AS ma_attribute, null col_a, null col_b
),h1 AS
(
SELECT
hk,
STRING_AGG(
REPLACE(
CONCAT(ISNULL(col_a,'^^'),'||',ISNULL(col_b,'^^')),
'^^||^^', '^^'
),'||') WITHIN GROUP (ORDER BY ma_attribute) hdiffstr,
LEFT(REPLICATE(CAST(@null_replacement+@delimiter AS varchar(max)),COUNT(*)),(LEN(@null_replacement+@delimiter)*COUNT(*))-LEN(@delimiter)) AS all_null
FROM source_data
GROUP BY hk
)
SELECT
h.*,
CASE
WHEN hdiffstr = all_null then CAST(REPLICATE(0x0,16) AS BINARY(16))
ELSE HASHBYTES('md5',h.hdiffstr)
END hdiff
FROM
h1 h