MDEV-31721: Cursor protocol increases the counter of "Empty_queries" for select

Problem:
  Empty queries are incremented if no rows are sent to the client in the
  EXECUTE phase of select query. With cursor protocol, rows are not sent
  during EXECUTE phase; they are sent later in FETCH phase. Hence,
  queries executed with cursor protocol are always falsely treated as
  empty in EXECUTE phase.

Fix:
  For cursor protocol, empty queries are now counted during the FETCH
  phase. This ensures counter correctly reflects whether any rows were
  actually sent to the client.

Tests included in `mysql-test/main/show.test`.

sql/sql_class.h

@@ -5850,6 +5850,17 @@ public:
       return false;
     return !is_set_timestamp_forbidden(this);
   }
+
+  /**
+    @brief
+    Return true if current statement uses cursor protocol for execution.
+
+    @details
+    Cursor protocol execution is determined by checking if lex->result is a
+    Select_materialize object, which is exclusively used by the server for
+    cursor result set materialization.
+  */
+  bool is_cursor_execution() const;
 };