[fix](agg) Aggregating string types with null values may result in incorrect result when compaction (#42512)

Follow-up for #42067.

Author: liaoxin01

be/src/vec/olap/vertical_block_reader.cpp

@@ -317,10 +317,10 @@ size_t VerticalBlockReader::_copy_agg_data() {
         auto& dst_column = _stored_data_columns[idx];
         if (_stored_has_variable_length_tag[idx]) {
             //variable length type should replace ordered
+            dst_column->clear();
             for (size_t i = 0; i < copy_size; i++) {
                 auto& ref = _stored_row_ref[i];
-                dst_column->replace_column_data(*ref.block->get_by_position(idx).column,
-                                                ref.row_pos, i);
+                dst_column->insert_from(*ref.block->get_by_position(idx).column, ref.row_pos);
             }
         } else {
             for (auto& it : _temp_ref_map) {