WITH a
         AS ( SELECT DISTINCT UNNEST(ARRAY['777', NULL]) "Ключ",
         UNNEST(ARRAY['111', '222']) "Знач"
         )
 update "Документы"
 set "Название" = "Название" || '_' || a."Знач"
 FROM a
 where "Id" IS NOT DISTINCT FROM  a."Ключ"
returning *

WITH a
         AS ( SELECT DISTINCT UNNEST(ARRAY['777', NULL]) "Ключ",
         UNNEST(ARRAY['333', '444']) "Знач"
         )
 update "Документы"
 set "Название" = "Название" || '_' || a."Знач"
 FROM a
 where
   EXISTS (
    SELECT TRUE
    FROM a
    WHERE "Id" = a."Ключ" and "Id" IS NOT NULL and a."Ключ" IS NOT NULL  
    UNION
     SELECT TRUE
    FROM a
    WHERE "Id" IS NULL and a."Ключ" IS NULL
   )
returning *