Only show the occupied icon for occupied cities

When using citybar style "simple", the "city is occupied" icon was always
shown. Only show it when the city is actually occupied.

Closes longturn#437.

client/tilespec.cpp

@@ -4908,7 +4908,8 @@ fill_sprite_array(struct tileset *t, enum mapview_layer layer,
                             FULL_TILE_Y_OFFSET + t->city_offset_y);
         }
       }
-      if (!citybar_painter::current()->has_units()) {
+      if (!citybar_painter::current()->has_units()
+          && pcity->client.occupied) {
         sprs.emplace_back(t,
                           get_city_sprite(t->sprites.city.occupied, pcity),
                           true, FULL_TILE_X_OFFSET + t->occupied_offset_x,