MDEV-34639 Check NULLness of field in ha_spider::field_exchange

When a view has more than one field, but we run an UPDATE statement on
it that involves only one field, then the sql layer may only populate
the field of that Item_field, and leave the others NULL. We therefore
add a check for the NULLness of the field.

storage/spider/ha_spider.cc

@@ -11418,6 +11418,8 @@ Field *ha_spider::field_exchange(
   Field *field
 ) {
   DBUG_ENTER("ha_spider::field_exchange");
+  if (!field)
+    DBUG_RETURN(NULL);
 #ifdef HA_CAN_BULK_ACCESS
   if (is_bulk_access_clone)
   {

storage/spider/mysql-test/spider/bugfix/r/mdev_34639.result

+INSTALL SONAME 'ha_spider';
+CREATE SERVER s_1 FOREIGN DATA WRAPPER mysql OPTIONS (
+HOST 'localhost',
+DATABASE 'auto_test_local',
+USER 'root',
+PASSWORD '',
+SOCKET '/home/ycp/source/mariadb-server/10.5/build/mysql-test/var/tmp/mysqld.1.1.sock'
+    );
+SET spider_internal_sql_log_off= 0;
+SET spider_same_server_link=on;
+CREATE SERVER srv FOREIGN DATA WRAPPER MYSQL OPTIONS (HOST "127.0.0.1", DATABASE "test", USER "root", PORT $MASTER_1_MYPORT);
+CREATE TABLE t1 (c1 INT,c2 INT) ENGINE=MyISAM;
+CREATE TABLE t2 (c1 INT,c2 INT) ENGINE=Spider COMMENT='WRAPPER "mysql", SRV "srv", TABLE "t1"';
+insert into t2 values (12, 31), (81, 90);
+CREATE VIEW v AS SELECT * FROM t2;
+UPDATE v SET c1=1;
+select * from t2;
+c1	c2
+1	31
+1	90
+DROP TABLE t1,t2;
+drop view v;
+DROP FUNCTION IF EXISTS spider_flush_table_mon_cache;
+DROP FUNCTION IF EXISTS spider_copy_tables;
+DROP FUNCTION IF EXISTS spider_ping_table;
+DROP FUNCTION IF EXISTS spider_bg_direct_sql;
+DROP FUNCTION IF EXISTS spider_direct_sql;
+UNINSTALL SONAME IF EXISTS 'ha_spider';
+Warnings:
+Warning	1620	Plugin is busy and will be uninstalled on shutdown
+DROP TABLE IF EXISTS mysql.spider_xa;
+DROP TABLE IF EXISTS mysql.spider_xa_member;
+DROP TABLE IF EXISTS mysql.spider_xa_failed_log;
+DROP TABLE IF EXISTS mysql.spider_tables;
+DROP TABLE IF EXISTS mysql.spider_link_mon_servers;
+DROP TABLE IF EXISTS mysql.spider_link_failed_log;
+DROP TABLE IF EXISTS mysql.spider_table_position_for_recovery;
+DROP TABLE IF EXISTS mysql.spider_table_sts;
+DROP TABLE IF EXISTS mysql.spider_table_crd;
+DROP SERVER IF EXISTS s_1;
+DROP SERVER IF EXISTS s_2_1;
+DROP SERVER IF EXISTS s_2_2;
+DROP SERVER IF EXISTS s_2_3;
+DROP SERVER IF EXISTS s_3_1;
+DROP SERVER IF EXISTS s_3_2;
+DROP SERVER IF EXISTS s_3_3;

storage/spider/mysql-test/spider/bugfix/t/mdev_34639.test

+--source ../../include/init_spider.inc
+SET spider_same_server_link=on;
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER MYSQL OPTIONS (HOST "127.0.0.1", DATABASE "test", USER "root", PORT $MASTER_1_MYPORT);
+CREATE TABLE t1 (c1 INT,c2 INT) ENGINE=MyISAM;
+CREATE TABLE t2 (c1 INT,c2 INT) ENGINE=Spider COMMENT='WRAPPER "mysql", SRV "srv", TABLE "t1"';
+insert into t2 values (12, 31), (81, 90);
+CREATE VIEW v AS SELECT * FROM t2;
+UPDATE v SET c1=1;
+select * from t2;
+# Cleanup
+DROP TABLE t1,t2;
+drop view v;
+--source ../../include/deinit_spider.inc