Detecting Unknown in automation

If I have a timed automation, what is the best way to stop it performing the action if a sensor has not got a value? If this triggers just after HA is started, and before the sensor has a value, it ends up passing ‘unknown’ to the service.

- id: Watchdog - Greenhouse Temperature
  alias: Watchdog - Greenhouse Temperature
  description: ''
  - platform: time_pattern
    minutes: /5
  - condition: template
    value_template: '{{ (as_timestamp(now()) - as_timestamp(states.sensor.propagator_temperature.last_updated)|int)
      > 270 }}'
  - service: shell_command.send_netlink
      key: extraTemp1
      value: '{{ states(''sensor.propagator_temperature'')}}'
  mode: single