fix: Home Assistant: guard value templates against missing properties

[deviantintegral]

I'm getting occasional errors when devices report with missing properties in Home Assistant. This PR should fix that!

I'm pushing the tests first which should fail, followed by the fix.

This PR also removes mockProperties based on the discussion below. Of note is that messages no longer contain properties with null values.

Before:

{
  action: "single",
  battery: null,
  identify: null,
  linkquality: null,
  voltage: null,
  power_outage_count: null,
  device_temperature: null,
}

After:

{
  action: "single"
}

[Koenkk]

• What value will HA show if no value is available?
• I like this approach more than the current mockProperties, I think this can fully replace the mockProperties?

[deviantintegral]

For most entities, the guard never fires. adjustMessageBeforePublish will inject the key with null, so prop is true and we render None. I think that gets mapped to [PAYLOAD_NONE](https://github.com/home-assistant/core/blob/d5b6bd957d47fa3cfacfee2884e18613d8d592f5/homeassistant/components/mqtt/const.py#L383), which leads to unknown. I don't think this is changed compared to dev.

For the ones without a matching mockProperty, the template renders "" and HA drops the message at a debug level. The entity should just show whatever the last value was.

So, the only visible effect is that the Jinja warnings stop. Hopefully!

I do think it's likely this could fully replace mockProperties, but that's probably a bigger change. Is that better as a separate PR or bundled in here?

[Koenkk]

Is that better as a separate PR or bundled in here?

If you see the chance, than I would prefer to bundle it here.

[deviantintegral]

Done! Let me know if you think we should preserve null values. I think it's a reasonable cleanup, but a strict reading would consider that a compatibility break.

[Koenkk]

great, I migrated the remaining cases, will merge this after the 1 October release such that we get enough test time.