$state and $homie can be empty

[daenney]

Based on the spec, it appears that both $state and $homie can be empty:

https://homieiot.github.io/specification/#device-attributes:

The following device attributes are mandatory and MUST be send, even if it is just an empty string. Topic	Description
$homie	The implemented Homie convention version
$name	Friendly name of the device
$state	See Device Lifecycle
$nodes	Nodes the device exposes, separated by , for multiple ones.
$extensions	Supported extensions, separated by , for multiple ones

If $homie is empty then I don't know which version of the convention the device implements, which complicates supporting different versions in the same network. I could make the assumption that empty means 4.0.0 but that seems wrong too. It feels like $homie should never be allowed to be empty. Even when a device is still in its initialising phase, it should already know which version of the convention it's going to publish itself as. The alternative would be to interpret a device with an empty $homie as the signal for "please forget about me" which would cause the controller to fully remove the device?

$state being empty also seems strange. The device lifecycle specifies a number of states a device can be in, and none of them is empty. It seems more practical that empty shouldn't be allowed and a device should default to init if it hasn't explicitly published a different state?

[Tieske]

good point. This also touches on #223 since empty strings mean deleting the topic in most MQTT servers.