Exporter: fix usage of nullptr, use ddl_nullptr instead.
Exporter: fix usage of nullptr, use ddl_nullptr instead.

kimkulling/openddl-parser,kimkulling/openddl-parser,kimkulling/openddl-parser,FredrikHson/openddl-parser,FredrikHson/openddl-parser,FredrikHson/openddl-parser,FredrikHson/openddl-parser,kimkulling/openddl-parser