MDEV-30870 Undo tablespace name displays wrongly for I_S queries

- INNODB_SYS_TABLESPACES in information schema should display
innodb_undo001, innodb_undo002 etc as tablespace name for undo
tablespaces

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

 SET GLOBAL innodb_undo_log_truncate = 0;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+=== information_schema.innodb_sys_tablespaces and innodb_sys_datafiles ===
+Space_Name	Page_Size	Zip_Size	Path
+innodb_undo001	DEFAULT	DEFAULT	MYSQLD_DATADIR//undo001
+innodb_undo002	DEFAULT	DEFAULT	MYSQLD_DATADIR//undo002
+innodb_temporary	DEFAULT	DEFAULT	MYSQLD_DATADIR/ibtmp1
 create table t1(keyc int primary key, c char(100)) engine = innodb;
 create table t2(keyc int primary key, c char(100)) engine = innodb;
 connect  con1,localhost,root,,;

mysql-test/suite/innodb/t/undo_truncate.opt

 --innodb-buffer-pool-size=24M
 --innodb-immediate-scrub-data-uncompressed=ON
+--loose-innodb-sys-tablespaces

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

@@ -15,6 +15,9 @@ call mtr.add_suppression("InnoDB: Trying to delete tablespace.*pending operation
 SET GLOBAL innodb_undo_log_truncate = 0;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
 
+LET $MYSQLD_DATADIR = `select @@datadir`;
+LET $INNODB_PAGE_SIZE = `select @@innodb_page_size`;
+--source suite/innodb/include/show_i_s_tablespaces.inc
 #-----------------------------------------------------------------------------
 #
 # Perform DML action using multiple clients and multiple undo tablespace.

storage/innobase/handler/i_s.cc

@@ -6436,8 +6436,13 @@ static int i_s_sys_tablespaces_fill(THD *thd, const fil_space_t &s, TABLE *t)
       OK(f->store(name.data(), name.size(), system_charset_info));
       f->set_notnull();
     }
-    else
-      f->set_notnull();
+    else if (srv_is_undo_tablespace(s.id))
+    {
+      char name[15];
+      snprintf(name, sizeof name, "innodb_undo%03zu",
+               (s.id - srv_undo_space_id_start + 1));
+      OK(f->store(name, strlen(name), system_charset_info));
+    } else f->set_notnull();
   }
 
   fields[SYS_TABLESPACES_NAME]->set_null();