You can use IsNull or Coalesce. Like this:
ALTER FUNCTION [dbo].[hmsGetLosText]
(@FromDt as datetime,@DateLeftOrg as Datetime)
returns varchar(255)
AS
BEGIN
DECLARE @yy AS SMALLINT, @mm AS INT, @dd AS INT,
@getmm as INT, @getdd as INT, @Fvalue varchar(255)
--if @DateLeftOrg is null, use GetDate
SET @DateLeftOrg = CASE WHEN @DateLeftOrg IS NULL THEN GetDate()
WHEN @DateLeftOrg = '1/1/1900' THEN GetDate()
ELSE @DateLeftOrg
END
SET @yy = DATEDIFF(yy, @FromDt, @DateLeftOrg)
SET @mm = DATEDIFF(mm, @FromDt, @DateLeftOrg)
SET @dd = DATEDIFF(dd, @FromDt, @DateLeftOrg)
SET @getmm = ABS(DATEDIFF(mm, DATEADD(yy, @yy, @FromDt), @DateLeftOrg))
SET @getdd = ABS(DATEDIFF(dd, DATEADD(mm, DATEDIFF(mm, DATEADD(yy, @yy, @FromDt), @DateLeftOrg)), DATEADD(yy, @yy, @FromDt)), @DateLeftOrg))
IF @getmm = 1
set @getmm=0
RETURN (
Convert(varchar(10),@yy) + 'y ' + Convert(varchar(10),@getmm) + 'm ')
END