create table #T (
ID int identity primary key,
XMLCol XML);

insert into #T (XMLCol)
select ' ';

insert into #T (XMLCol)
select '';

insert into #T
default values;

select *
from #T
where XMLCol is not null and cast(XMLCol as varchar(max)) != '';

DECLARE @var XML

SET @var = '' -- '<test>test</test>'

SELECT  @var.exist('/test')

IF @var IS NOT NULL  AND  @var.exist('/test') = 1
    BEGIN
        SELECT 'not null'
    END
 ELSE
    BEGIN
		SELECT 'null'
	END