Automation variable occasionally 'unavailable'

1

I have an automation that when triggered calls a shell command, passing the new value of the sensor. Nine out of ten times this works, but occasionally the shell command, instead of sending a temperature, sends the string ‘unavailable’. Is there anyway I can put a conditional in the HA shell command, so it does nothing rather that sending unavailable?

This is the automation:

- id: Relay Greenhouse Temperature
  alias: Relay Greenhouse Temperature
  description: Send greenhouse temperature onto Weewx via Netlink
  trigger:
  - platform: state
    entity_id: sensor.propagator_temperature
  condition:
  - condition: numeric_state
    entity_id: sensor.propagator_temperature
    above: '-5'
    below: '50'
  action:
  - service: shell_command.send_netlink
    data:
      key: extraTemp1
      value: '{{ trigger.to_state.state }}'

And this is the shell command.

shell_command:
 send_netlink: /bin/bash -c "echo -e {{ key }}:{{ value }}':' | nc 192.168.98.10 2029"