Page.java

   @Property
   private Country country;
   
   @Property
   private City city;
   
   public Country[] getCountries() {
      return new Country[] { ... };
   }
   
   public List<City> getNonEmptyCities() {
      if (country.getCities().isEmpty()) {
        return Collections.singletonList(new City(null));
      }
      return country.getCities();
   }
   
   public List<Street> getNonEmptyStreets() {
     if (city.getStreets().isEmpty()) {
       return Collections.singletonList(new Street(null, 0)
     }
     return city.getStreets();
   }


}

Page.tml

   <t:grid source="countries" row="country" add="cities" t:mixins="stitch/GridCollapse" collapseColumn="cities">
      <p:citiesCell>
         <t:grid source="nonEmptyCities" row="city" add="streets" t:mixins="stitch/GridCollapse,stitch/DisableSort" collapseColumn="streets">
            <p:streetsCell>
               <t:grid source="nonEmptyStreets" t:mixins="stitch/DisableSort" />
            </p:streetsCell>
         </t:grid>
      </p:citiesCell>
   </t:grid>