MDEV-28255 "Error" instead of NULL in P_S.THREADS_CONNECTION_TYPE for background threads

use vio_type_names[] values as in MySQL

mysql-test/suite/perfschema/r/threads_mysql.result

 SET GLOBAL event_scheduler = OFF;
 SELECT name, type, processlist_user, processlist_host, processlist_db,
-processlist_command, processlist_info,
+processlist_command, processlist_info, connection_type,
 IF(parent_thread_id IS NULL, parent_thread_id, 'unified parent_thread_id')
 AS unified_parent_thread_id,
 role, instrumented
@@ -14,6 +14,7 @@ processlist_host	NULL
 processlist_db	mysql
 processlist_command	NULL
 processlist_info	NULL
+connection_type	NULL
 unified_parent_thread_id	NULL
 role	NULL
 instrumented	YES
@@ -24,6 +25,7 @@ processlist_host	NULL
 processlist_db	NULL
 processlist_command	NULL
 processlist_info	NULL
+connection_type	NULL
 unified_parent_thread_id	unified parent_thread_id
 role	NULL
 instrumented	YES
@@ -34,13 +36,14 @@ processlist_host	localhost
 processlist_db	test
 processlist_command	Query
 processlist_info	SELECT name, type, processlist_user, processlist_host, processlist_db,
-processlist_command, processlist_info,
+processlist_command, processlist_info, connection_type,
 IF(parent_thread_id IS NULL, parent_thread_id, 'unified parent_thread_id')
 AS unified_parent_thread_id,
 role, instrumented
 FROM performance_schema.threads
 WHERE name LIKE 'thread/sql%'
 ORDER BY name
+connection_type	Socket
 unified_parent_thread_id	unified parent_thread_id
 role	NULL
 instrumented	YES
@@ -51,6 +54,7 @@ processlist_host	NULL
 processlist_db	NULL
 processlist_command	NULL
 processlist_info	NULL
+connection_type	NULL
 unified_parent_thread_id	unified parent_thread_id
 role	NULL
 instrumented	YES

mysql-test/suite/perfschema/t/threads_mysql.test

@@ -32,7 +32,7 @@ SET GLOBAL event_scheduler = OFF;
 #    Therefore we have to disable this protocol for the next statement.
 --disable_ps_protocol
 SELECT name, type, processlist_user, processlist_host, processlist_db,
-       processlist_command, processlist_info,
+       processlist_command, processlist_info, connection_type,
        IF(parent_thread_id IS NULL, parent_thread_id, 'unified parent_thread_id')
          AS unified_parent_thread_id,
        role, instrumented

vio/viosocket.c

@@ -647,7 +647,7 @@ enum enum_vio_type vio_type(Vio* vio)
 
 static const LEX_CSTRING vio_type_names[] =
 {
-  { STRING_WITH_LEN("Error") }, // cannot happen
+  { STRING_WITH_LEN("") }, // internal threads
   { STRING_WITH_LEN("TCP/IP") },
   { STRING_WITH_LEN("Socket") },
   { STRING_WITH_LEN("Named Pipe") },