You could do the following:
EsriRuntimeQt::SpatialReference sr(102100)
EsriRuntimeQt::Point new_point = EsriRuntimeQt::CoordinateConversion::DecimalDegreesToPoint(longitude, sr);
This gives me valid coordinates. My guess is that your map isn't ready (i.e. there is no spatial reference set yet, because it isn't finished initializing), so because there is no valid sr going into the function, it comes back as NaN. You can use the onMapReady slot and wait to do this calculation until that point, or you could try what I did above.
-Luke