SQL Server 2019

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CREATE TABLE used_listings_v2 
(
   id int 
   , image varchar(50)
   , image_b varchar(50)
   , image_c varchar(50)
   , image_d varchar(50)
)
;

INSERT INTO used_listings_v2 (id, image, image_b, image_c, image_d)
VALUES (1, 'arrow.png', 'up.png', 'down.png', '')
;

1 rows affected

SELECT 'Before', *
FROM    used_listings_v2


(No column name)
id
image
image_b
image_c
image_d


Before
1
arrow.png
up.png
down.png



UPDATE used_listings_v2
SET    image = CASE WHEN LEN(image) = 0 THEN NULL ELSE image END
       , image_b = CASE WHEN LEN(image_b) = 0 THEN NULL ELSE image_b END
       , image_c = CASE WHEN LEN(image_c) = 0 THEN NULL ELSE image_c END
       , image_d = CASE WHEN LEN(image_d) = 0 THEN NULL ELSE image_d END
WHERE id = 1
1 rows affected

SELECT 'After', *
FROM    used_listings_v2


(No column name)
id
image
image_b
image_c
image_d


After
1
arrow.png
up.png
down.png
null