Hi,
You can use an expression with an IF condition on your line to fix this. First, remove the space between your salutation and FirstName:
@Salutation@@Firstname@ @last name@
Then, edit your @salutation@ variable to the following:
IF(TRIM([Salutation]) <> "", [Salutation] & " ", "")
This will basically check if your salutation data field is empty. If it's not, it prints the salutation with a space. If it's empty, it prints nothing ("" being an empty string).
Hope this helps,
Eric.