MDEV-27993 Assertion failed in btr_page_reorganize_low()

btr_cur_optimistic_insert(): Disregard DEBUG_DBUG injection to
invoke btr_page_reorganize() if the page (and the table) is empty.
Otherwise, an assertion would fail in btr_page_reorganize_low()
because PAGE_MAX_TRX_ID is 0 in an empty secondary index leaf page.

mysql-test/suite/innodb/r/page_reorganize.result

@@ -25,3 +25,10 @@ f1
 disconnect con1;
 connection default;
 drop table t1;
+#
+# MDEV-27993 Assertion failed in btr_page_reorganize_low()
+#
+CREATE TABLE t1(a INT PRIMARY KEY, b INT UNIQUE) ENGINE=InnoDB;
+SET DEBUG_DBUG = '+d,do_page_reorganize';
+INSERT INTO t1 VALUES(0,0);
+DROP TABLE t1;

mysql-test/suite/innodb/t/page_reorganize.test

@@ -53,4 +53,12 @@ connection default;
 
 drop table t1;
 
+--echo #
+--echo # MDEV-27993 Assertion failed in btr_page_reorganize_low()
+--echo #
+CREATE TABLE t1(a INT PRIMARY KEY, b INT UNIQUE) ENGINE=InnoDB;
+SET DEBUG_DBUG = '+d,do_page_reorganize';
+INSERT INTO t1 VALUES(0,0);
+DROP TABLE t1;
+
 --source include/wait_until_count_sessions.inc

storage/innobase/btr/btr0cur.cc

@@ -3,7 +3,7 @@
 Copyright (c) 1994, 2019, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2015, 2021, MariaDB Corporation.
+Copyright (c) 2015, 2022, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3212,6 +3212,7 @@ btr_cur_optimistic_insert(
 		 << ib::hex(thr ? thr->graph->trx->id : 0)
 		 << ' ' << rec_printer(entry).str());
 	DBUG_EXECUTE_IF("do_page_reorganize",
+			if (n_recs)
 			btr_page_reorganize(page_cursor, index, mtr););
 
 	/* Now, try the insert */