branch-2.1: [Optimize](Expr) Opt getting value of VLitreal (#43249)

Cherry-picked from #43204

Co-authored-by: zclllhhjj <zhaochangle@selectdb.com>

be/src/vec/exprs/vliteral.cpp

@@ -76,15 +76,8 @@ Status VLiteral::execute(VExprContext* context, vectorized::Block* block, int* r
 }
 
 std::string VLiteral::value() const {
-    //TODO: dcheck the equality of size with 1. then use string with size to replace the ss.
-    std::stringstream out;
-    for (size_t i = 0; i < _column_ptr->size(); i++) {
-        if (i != 0) {
-            out << ", ";
-        }
-        out << _data_type->to_string(*_column_ptr, i);
-    }
-    return out.str();
+    DCHECK(_column_ptr->size() == 1);
+    return _data_type->to_string(*_column_ptr, 0);
 }
 
 std::string VLiteral::debug_string() const {