ifcgeomserver: test iterator->next() via operator bool (fix ambiguous !=)

Iterator::next() now returns express::Base (data-model branch). Comparing
it against 0 is ambiguous: 0 converts to Base via the pointer ctor while
Base converts to int via operator bool, so both operator!=(int,int) and
Base::operator!= are candidates. Use an explicit truthiness test — an
empty Base signals end-of-iteration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcgeomserver/IfcGeomServer.cpp

@@ -629,7 +629,7 @@ int main () {
 		}
 		case NEXT: {
 			Next n; n.read(std::cin);
-			has_more = iterator->next() != 0;
+			has_more = static_cast<bool>(iterator->next());
 			if (!has_more) {
 				delete file;
 				delete iterator;