Prova det med Coalesce .
$query = " UPDATE user
SET
`Title` = CASE WHEN `Title`='' or `Title` IS NULL THEN '$title' END,
`Description` = CASE WHEN `Description`='' Or `Description` IS NULL THEN '$description' END,
`Date` = CASE WHEN `Date`='' Or Date` IS NULL THEN '$date' END
WHERE `id` = '".$id."' ";
eller :
$query = " UPDATE user
SET
`id` = Coalesce('$id''".$id."' , NULLIF(`id`,'')),
`Title` = Coalesce('$title''".$title."',NULLIF(`Title`,'') ) ,
`Description` = Coalesce('$description''".$description."' , NULLIF(`Description`,'') ) ,
`Date` = Coalesce('$date''".$date."',NULLIF(`Date`,''))
WHERE `id` = '$id''".$id."' ";