[fix](virtual slot) Fix some expression can not process empty block

be/src/olap/rowset/segment_v2/segment_iterator.cpp

@@ -2960,6 +2960,17 @@ void SegmentIterator::_init_virtual_columns(vectorized::Block* block) {
 
 Status SegmentIterator::_materialization_of_virtual_column(vectorized::Block* block) {
     size_t prev_block_columns = block->columns();
+    // Some expr can not process empty block, such as function `element_at`.
+    // So materialize virtual column in advance to avoid errors.
+    if (block->rows() == 0) {
+        for (const auto& pair : _vir_cid_to_idx_in_block) {
+            auto& col_with_type_and_name = block->get_by_position(pair.second);
+            col_with_type_and_name.column = _opts.vir_col_idx_to_type[pair.second]->create_column();
+            col_with_type_and_name.type = _opts.vir_col_idx_to_type[pair.second];
+        }
+        return Status::OK();
+    }
+
     for (const auto& cid_and_expr : _virtual_column_exprs) {
         auto cid = cid_and_expr.first;
         auto column_expr = cid_and_expr.second;

regression-test/data/query_p0/virtual_slot_ref/return_in_advance_if_block_is_empty.out

@@ -0,0 +1,3 @@
+-- This file is automatically generated. You should know what you did if you want to edit this
+-- !sql_1 --
+