sqapi: check whether supplied string parameters are not null

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6386 8aca7d54-2c30-db11-9de9-000461428c89
sqapi: interface will not change until 112.2 release

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6387 8aca7d54-2c30-db11-9de9-000461428c89