Without the is_defined filter home assistant will reset the value to an empty string every time a topic is posted without the value template ie when `home/OpenMQTTGateway/BTtoMQTT/--SNIP-- {"moi":2}` is posted - all other sensor with associated topic `home/OpenMQTTGateway/BTtoMQTT/--SNIP--` will get `""` as their value
