There are a couple of other points here.
Firstly, if any of the fields being concatenated 'contains' null, the resultant string is null, so it may me useful to coalesce the fields first (i.e. change Null to Space).
Secondly, it could be useful to RTRIM the fields to remove any trailing spaces ion each element, and then to put one space back as was suggested.
rtrim(coalesce(TABLE.FIRSTNAME,' ')||' '||coalesce(TABLE.SURNAME,' ')