Fix disabling HA discovery after device was discovered

[nurikk]

This fixes rare case when device was published in HA discovery, and then through configuration.yaml was set homeassistant: null (to disable this device discovery)

Here is workflow:

1. I have discovered device
2. I want to kick this device from hass discovery
3. I set device specific config homeassistant: null
4. Restart z2m
5. Device was removed from discovery

0|zigbee2mqtt  | Zigbee2MQTT:error 2020-12-05 13:09:22: Failed to call 'HomeAssistant' 'onMQTTMessage' (TypeError: Cannot read property 'light' of null
0|zigbee2mqtt  |     at /home/nur/.nvm/versions/node/v12.18.3/lib/node_modules/zigbee2mqtt/lib/extension/homeassistant.js:616:77
0|zigbee2mqtt  |     at Array.forEach (<anonymous>)