In order to accept and return differing data types, you'd have to use sql_variant, which will affect how easily you can use the value returned by the function:

CREATE FUNCTION [dbo].[fnchkNull] (

@colname sql_variant

)

RETURNS sql_variant

AS

BEGIN

RETURN (

SELECT CASE

WHEN CAST(SQL_VARIANT_PROPERTY ( @colname , 'BaseType' ) AS varchar(30)) LIKE '%int%'

THEN ISNULL(@colname, 0)

WHEN @colname IS NULL OR @colname = '' OR @colname = 'NULL'

THEN 'No Data'

ELSE @colname

END

)

END