SQL - is not null syntax help - SQL Server 2005

Hello experts,
I have the below snippet of code that is causing me troubles.

declare @HGA1C varchar(30)
set @HGA1C= ''

if (@HGA1C is not null or @HGA1C <> '')
set @HGA1C = CONVERT(VARCHAR(10), convert(datetime,@HGA1C, 101),101 )

select @HGA1C

When I run the above code it returns '01/01/1900'

However, I would like it to simply return a blank segment.  Is there any way to code around this?  My problem is that most of the time this @HGA1C variable HAS a value, but sometimes it is blank - and my if statement above almost seems to be ignored.  

Thanks!