Steps to reproduce:

1. Use standart template https://github.com/zabbix/zabbix/tree/master/templates/db/mysql
2. mysql -e "create database test1;"
3. mysql -e "SELECT SUM(DATA_LENGTH + INDEX_LENGTH) FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA='test1'"
4. Return NULL
5. See latest data in web-frontend, item "Size of database test1" is not value

Fix:

Change in template_db_mysql.conf on:
UserParameter=mysql.dbsize[*], mysql -h"$1" -P"$2" -sN -e "SELECT IFNULL(SUM(DATA_LENGTH + INDEX_LENGTH),0) FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA='$3'"