[compability](variant) fix nullable serde during upgrading

need to check nullable during serialization when upgrading to 3.0

be/src/vec/columns/column_object.cpp

@@ -1150,7 +1150,12 @@ Status find_and_set_leave_value(const IColumn* column, const PathInData& path,
                 "failed to set value for path {}, expected type {}, but got {} at row {}",
                 path.get_path(), type->get_name(), column->get_name(), row);
     }
-    const auto* nullable = assert_cast<const ColumnNullable*>(column);
+    const auto* nullable = check_and_get_column<ColumnNullable>(column);
+    if (!nullable) {
+        return Status::InternalError("failed to set value for path {}, expected nullable column, but "
+                                     "got {} at row {}",
+                                     path.get_path(), column->get_name(), row);
+    }
     if (nullable->is_null_at(row) || (path.empty() && nullable->get_data_at(row).empty())) {
         return Status::OK();
     }