SQL Server adding values

SELECT @s = ISNULL(@s + '开发者_Python百科. ' , '') + [Comments] FROM [Customer]

RETURN @s   

How can I catch Null or '', so that if its either It Won't append to the string and not add a '.'

Thanks

Jacob

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SELECT
    @s = ISNULL(NULLIF(@s, '') + '.', '') + comments
FROM
    [Customers]

RETURN @s