Fix migration on some SQL systems

[kemenaran]

Fix this issue on Raspberry Pi:

ynh_mysql_execute_as_root '--sql=SELECT CONCAT( '\''DROP TABLE '\'', GROUP_CONCAT(table_name) , '\'';'\'' ) AS statement FROM information_schema.tables WHERE table_schema = '\''mattermost'\'' AND table_name LIKE '\''focalboard_%'\'';' --database=mattermost
WARNING - ERROR 1064 (42000) at line 1: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'NULL' at line 1

[kemenaran]

!testme

[yunohost-bot]

Fingers crossed!