you need to convert/cast it each time. I made a function to use:

CREATE FUNCTION QuoteNull
(
     @InputStr      varchar(8000)  --value to force to string
)
RETURNS
varchar(8000)
AS

BEGIN
    RETURN COALESCE(''''+@InputStr+'''','null')
END

it puts single quotes around the value or just the word null if it is null, but you can customize it as necessary.

here is a version that handles formatting dates automatically:

CREATE FUNCTION QuoteNull
(
     @InputStr      sql_variant   --value to force to string
)
RETURNS
varchar(8000)
AS

BEGIN
    DECLARE @String  varchar(8000)
    SET @String=COALESCE(''''+  CASE SQL_VARIANT_PROPERTY(@InputStr,'BaseType')
                                    WHEN 'datetime' THEN CONVERT(varchar(23),@InputStr,121)
                                    ELSE CONVERT(varchar(8000),@InputStr)
                                END
                             +'''','null')
    RETURN @String
END