SQL Server: WHERE clause to ignore blank GUID

SELECT ...
FROM ...
WHERE CASE WHEN @ActivityUID = '00000000-0000-0000-0000-00000开发者_如何学JAVA0000000' 
    THEN
        -- do nothing
    ELSE
        (Activity.ActivityUID = @ActivityUID)
    END

When a blank Guid is passed in via ActivityUID then I want the WHERE statement to ignore Activity.ActivityUID = @ActivityUID

WHERE (
         Activity.ActivityUID = @ActivityUID OR 
         @ActivityUID = '00000000-0000-0000-0000-000000000000'
      )

This removes the need for an OR clause because NULL comparisons fail.

WHERE
    Activity.ActivityUID = NULLIF(@ActivityUID, '00000000-0000-0000-0000-000000000000')

See if there is a performance difference...