SELECT WhatEver
FROM MyTable
Where Something = X
ORDER BY ISNULL(StringColmn,'zzzzz')

SELECT WhatEver
FROM MyTable
WHERE Something = X
ORDER BY CASE WHEN StringColumn IS NULL THEN 1 ELSE 0 END, StringColumn

declare @DemoTbl table (i int identity(1,1), ch char(1))

insert into @DemoTbl (ch)
select 'b'
union all select null
union all select 'a'
union all select null
union all select 'c'
union all select 'd'

select *
from @DemoTbl
order by case when ch is null then 1 else 0 end, ch

select * 
from @DemoTbl
order by ISNULL(ch,'zzzz'), ch