INSERT INTO FROM: Concatenating Forename and Surname when Forename can be blank,
Liya has a nice solution. This is another that handles both the null case and the empty string last name case as well. 

INSERT INTO [dbo].Test
select LTRIM(RTRIM (COALESCE(firstname,'') + ' ' + COALESCE(lastname,'')))
    ,field1
    ,field2
    ,fieldn
FROM RAW.Test ;


EDIT: Modified to allow either field to be nullable or an empty string with trimming in full effect in response to the modified requirements of the OP.