As you have seen in the article SQL Concatenating: NULL problem you may have problems concatenating a set values if one of them contains a NULL value. Another problem might be if you concat values of different type.
SELECT CAST(1 as nvarchar)+'st TE'+null+'ST' --Results in NULL SELECT 1+'st TE'+null+'ST' --results in InvalidCastException from varchar to int.
Beginning with the new SQL Server 2012 there is a new function called CONCAT that concatenates a bunch of objects. The cool feature is that there can be concatenated object of different type like NVARCHAR, INT, FLOAT, etc. You can also concatenate NULL without getting NULL as result. Additional you must not to convert the values to nvarchar prior passing as parameter to the CONCAT function.
SELECT CONCAT(1,'st TE',null,'ST') --Results in 1st TEST