Default value when value_json.XXX is "unknown" for mqtt based sensor

I have a template sensor where the value_template is set to extract the value from value_json… however since this is a MQTT based sensor it is possible that there has been no value after a restart of the system. I am wondering how to set a default value here if there is not yet a value set through mqtt. I don’t necessarily want a retained value here since this sensor is not actually always available, so for me it makes more sense to default it to 0. Sensor definition is below.

sensor:
  - platform: mqtt
    name: "Smoker Temp 1"
    state_topic: "smoker/temp1/temperature"
    value_template: "{{ value_json.value }}"
    unit_of_measurement: 'F'
value_template: >
  {% if value_json.value = none %}
    0
  {% else %}
    {{ value_json.value }}
  {% endif %}

Thanks… tried this but the sensor state is still ‘unknown’ after using this

What about

{% if value_json is defined %}
  {{ value_json.value }}
{% else %}
  0
{% endif %}