create table record_description
( id int
, name varchar(30)
, description varchar(255)
, primary key(id)
);
insert into record_description
( values row( 1, 'a', 'test:alt="".')
       , row( 2, 'b', null)
       , row( 3, 'c', 'other')
);
-- v1
update record_description
set `description` = replace( `description`
                           , 'alt=""'
                           , concat( 'alt="', `name`, '"')
                           )
where `description` like '%alt=""%'
;
-- v2
update record_description
set `description` = replace
                    (   replace( `description`
                               , 'alt=""'
                               , 'alt="[STR-HOLDER]"'
                               )
                    , '[STR-HOLDER]'
                    , `name`
                    )
where `description` like '%alt=""%'
;