message: Return "empty coarse" if there are no spheres/gaussians in IHM model
diff: @@ -28,6 +28,8 @@ export const EmptyIHMCoarse = { hierarchy: CoarseHierarchy.Empty, conformation:
  export function getIHMCoarse(data: IHMData): { hierarchy: CoarseHierarchy, conformation: CoarseConformation } {
  const { ihm_sphere_obj_site, ihm_gaussian_obj_site } = data;
  + if (ihm_sphere_obj_site._rowCount === 0 && ihm_gaussian_obj_site._rowCount === 0) return EmptyIHMCoarse;
  +
  const sphereData = getData(ihm_sphere_obj_site);
  const sphereConformation = getSphereConformation(ihm_sphere_obj_site);
  const sphereKeys = getCoarseKeys(sphereData, data.entities);
  