Use unknown as the "missing" HASS MQTT Discovery state payload

[bachya]

Describe what the PR does:

When encountering a data point with no value, the Home Assistant MQTT Discovery publisher:

1. Sends a state value of None
2. Marks the entity as unavailable

It turns out that None will cause other HASS integrations (like Google Assistant) to fail. So, this PR makes the following changes:

1. Entities are always marked as available (since we have no way of knowing things like whether the gateway is connected).
2. When a data point has no value, we send unknown (a valid value).

This is technically a breaking change since downstream HASS users will need to be aware that entities (a) will now always be available and (b) plan for an unknown state if appropriate (e.g., for something like wind chill, where the value exists only under certain circumstances).

Does this fix a specific issue?

Fixes #293

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.