General tiles with only BUILDING_LIST

Bug found during code-read.  If tile just contained BUILDING_LIST
entries (i.e. no OBJECT_STATIC or OBJECT_SIGN entries), then it would
not generate.

simgear/scene/tgdb/ReaderWriterSTG.cxx

@@ -586,7 +586,7 @@ struct ReaderWriterSTG::_ModelBin {
             i->_elev += elevation(*terrainGroup, SGGeod::fromDeg(i->_lon, i->_lat));
         }
 
-        if (_objectStaticList.empty() && _signList.empty()) {
+        if (_objectStaticList.empty() && _signList.empty() && (_buildingListList.size() == 0)) {
             // The simple case, just return the terrain group
             return terrainGroup.release();
         } else {