MDEV-24757 : fix potential null pointer dereference in I_S.thread_pool_queues

With the fix, new connection without THDs will now show NULL in
thread_pool_queues.CONNECTION_ID.

mysql-test/main/thread_pool_info.result

@@ -31,7 +31,7 @@ Field	Type	Null	Key	Default	Extra
 GROUP_ID	int(6)	NO		0	
 POSITION	int(6)	NO		0	
 PRIORITY	int(1)	NO		0	
-CONNECTION_ID	bigint(19) unsigned	NO		0	
+CONNECTION_ID	bigint(19) unsigned	YES		NULL	
 QUEUEING_TIME_MICROSECONDS	bigint(19)	NO		0	
 DESC INFORMATION_SCHEMA.THREAD_POOL_STATS;
 Field	Type	Null	Key	Default	Extra

sql/thread_pool_info.cc

@@ -92,7 +92,7 @@ static ST_FIELD_INFO queues_field_info[] =
   Column("GROUP_ID",                   SLong(6),      NOT_NULL),
   Column("POSITION",                   SLong(6),      NOT_NULL),
   Column("PRIORITY",                   SLong(1),      NOT_NULL),
-  Column("CONNECTION_ID",              ULonglong(19), NOT_NULL),
+  Column("CONNECTION_ID",              ULonglong(19), NULLABLE),
   Column("QUEUEING_TIME_MICROSECONDS", SLonglong(19), NOT_NULL),
   CEnd()
 };
@@ -130,6 +130,7 @@ static int queues_fill_table(THD* thd, TABLE_LIST* tables, COND*)
         /* PRIORITY */
         table->field[2]->store(prio, true);
         /* CONNECTION_ID */
+        if (c->thd)
           table->field[3]->store(c->thd->thread_id, true);
         /* QUEUEING_TIME */
         table->field[4]->store(now - c->enqueue_time, true);