update artifact_type column alteration (goharbor#20239)

update column if is null

Signed-off-by: yminer <yminer@vmware.com>
Co-authored-by: Wang Yan <wangyan@vmware.com>
Signed-off-by: wang yan <wangyan@vmware.com>

make/migrations/postgresql/0140_2.11.0_schema.up.sql

@@ -20,12 +20,12 @@ table artifact:
 /*
 Add new column artifact_type for artifact table to work with oci-spec v1.1.0 list referrer api
 */
-ALTER TABLE artifact ADD COLUMN artifact_type varchar(255);
+ALTER TABLE artifact ADD COLUMN IF NOT EXISTS artifact_type varchar(255);
 
 /*
 set value for artifact_type
 then set column artifact_type as not null
 */
-UPDATE artifact SET artifact_type = media_type;
+UPDATE artifact SET artifact_type = media_type WHERE artifact_type IS NULL;
 
 ALTER TABLE artifact ALTER COLUMN artifact_type SET NOT NULL;