Assertion failure upon DECODE_HISTOGRAM with NULLs in first column

CREATE TABLE t1 (a INT, b INT);

INSERT INTO t1 VALUES (NULL,1), (NULL,2);

SET histogram_type = JSON_HB;

ANALYZE TABLE t1 PERSISTENT FOR ALL;

SELECT DECODE_HISTOGRAM(hist_type, histogram) from mysql.column_stats;

# Cleanup

DROP TABLE t1;

mariadbd: /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_type.cc:7457: bool Type_handler::Item_send_str(Item*, Protocol*, st_value*) const: Assertion `!it

em->null_value' failed.

210911 12:44:49 [ERROR] mysqld got signal 6 ;

#7  0x00007fcbe9582f36 in __GI___assert_fail (assertion=0x5595b90af58f "!item->null_value", file=0x5595b90ac490 "/data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_type.cc", line=7457, function=0x5595b90af5a8 "bool Type_handler::Item_send_str(Item*, Protocol*, st_value*) const") at assert.c:101

#8  0x00005595b7c7dcc9 in Type_handler::Item_send_str (this=0x5595ba6f9ac0 <type_handler_long_blob>, item=0x7fcbc8015a90, protocol=0x7fcbc80013a0, buf=0x7fcbe45ddc60) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_type.cc:7457

#9  0x00005595b7ad0358 in Type_handler_string_result::Item_send (this=0x5595ba6f9ac0 <type_handler_long_blob>, item=0x7fcbc8015a90, protocol=0x7fcbc80013a0, buf=0x7fcbe45ddc60) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_type.h:5440

#10 0x00005595b74ca666 in Item::send (this=0x7fcbc8015a90, protocol=0x7fcbc80013a0, buffer=0x7fcbe45ddc60) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/item.h:1227

#11 0x00005595b754fe9e in Protocol::send_result_set_row (this=0x7fcbc80013a0, row_items=0x7fcbc8015618) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/protocol.cc:1327

#12 0x00005595b76b6e27 in select_send::send_data (this=0x7fcbc8016bd0, items=...) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_class.cc:3072

#13 0x00005595b7914c19 in select_result_sink::send_data_with_check (this=0x7fcbc8016bd0, items=..., u=0x7fcbc8005120, sent=1) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_class.h:5634

#14 0x00005595b78e34fa in end_send (join=0x7fcbc8016bf8, join_tab=0x7fcbc8018278, end_of_records=false) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:22285

#15 0x00005595b78de5b1 in evaluate_join_record (join=0x7fcbc8016bf8, join_tab=0x7fcbc8017ec8, error=0) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:21278

#16 0x00005595b78dda54 in sub_select (join=0x7fcbc8016bf8, join_tab=0x7fcbc8017ec8, end_of_records=false) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:21094

#17 0x00005595b78dbe49 in do_select (join=0x7fcbc8016bf8, procedure=0x0) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:20602

#18 0x00005595b788b47c in JOIN::exec_inner (this=0x7fcbc8016bf8) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:4735

#19 0x00005595b78897a5 in JOIN::exec (this=0x7fcbc8016bf8) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:4513

#20 0x00005595b788c97b in mysql_select (thd=0x7fcbc8000db8, tables=0x7fcbc8015c10, fields=..., conds=0x0, og_num=0, order=0x0, group=0x0, having=0x0, proc_param=0x0, select_options=2147748608, result=0x7fcbc8016bd0, unit=0x7fcbc8005120, select_lex=0x7fcbc8015378) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:4991

#21 0x00005595b786cbb8 in handle_select (thd=0x7fcbc8000db8, lex=0x7fcbc8005058, result=0x7fcbc8016bd0, setup_tables_done_option=0) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_select.cc:545

#22 0x00005595b77c7176 in execute_sqlcom_select (thd=0x7fcbc8000db8, all_tables=0x7fcbc8015c10) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_parse.cc:6256

#23 0x00005595b77b342a in mysql_execute_command (thd=0x7fcbc8000db8, is_called_from_prepared_stmt=false) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_parse.cc:3946

#24 0x00005595b77d086c in mysql_parse (thd=0x7fcbc8000db8, rawbuf=0x7fcbc8015290 "SELECT DECODE_HISTOGRAM(hist_type, histogram) from mysql.column_stats", length=69, parser_state=0x7fcbe45df480) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_parse.cc:8030

#25 0x00005595b77a6b0c in dispatch_command (command=COM_QUERY, thd=0x7fcbc8000db8, packet=0x7fcbc800b849 "", packet_length=69, blocking=true) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_parse.cc:1896

#26 0x00005595b77a3ba2 in do_command (thd=0x7fcbc8000db8, blocking=true) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_parse.cc:1404

#27 0x00005595b7b0d493 in do_handle_one_connection (connect=0x5595bc8dd8c8, put_in_cache=true) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_connect.cc:1418

#28 0x00005595b7b0cc5b in handle_one_connection (arg=0x5595bc8dc1f8) at /data/src/preview-10.7-MDEV-21130-json-histograms/sql/sql_connect.cc:1312

#29 0x00005595b83eea83 in pfs_spawn_thread (arg=0x5595bc8dd418) at /data/src/preview-10.7-MDEV-21130-json-histograms/storage/perfschema/pfs.cc:2201

#30 0x00007fcbe9a9b609 in start_thread (arg=<optimized out>) at pthread_create.c:477

#31 0x00007fcbe966e293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95