schema for _type=lwt is requiring data

[jpmens]

As reported in owntracks/ios#755 (but likely not the reason for the crash):

2023-12-15T14:37:58.693Z  Validation error: Error Domain=com.argentumko.JSONSchemaValidationError Code=300 "JSON instance validation against the schema failed." UserInfo={path=, object={
  "_type" : "lwt",
  "tst" : 1702479180
}, NSLocalizedDescription=JSON instance validation against the schema failed., validator=<DSJSONSchemaObjectValidator: 0x2820b7260>{ maximum properties: none, minimum properties: 0, required properties: _type, data }, NSLocalizedFailureReason=Object is missing required properties: 'data'.} with {"_type":"lwt","tst":1702479180}

[ckrey]

This is probably when using application level encryption. I guess lwt is not encrypted

[jpmens]

Looking at #755 again regarding the not-encrypted lwt message: I think it's fine and logical that it's not encrypted (and it can't really be as the broker doesn't know anything about encryption), but I wonder whether it would be feasable to give the broker an LWT to transmit which has encrypted data ... ? IIRC libsodium isn't time sensitive, so technically it ought to be possible to

1. encrypt a payload for LWT
2. submit the LWT on connect to the broker
3. have the broker transmit the pre-encrypted LWT when the client dies

I'm definitely not saying we should do this; I'm rather thinking out loud whether it could make sense?

A disadvantage would be that monitors (other than, say, Recorder) for LWT would not be able to decrypt the payload.

[yantoz]

This is probably when using application level encryption. I guess lwt is not encrypted

@ckrey yes, application level encryption is enabled.

[ckrey]

I learned a lot about how app level encryption is used with OwnTracks...

[ckrey]

Basically, old versions process encrypted and unencrypted messages well.
In the newer versions, non encrypted messages cause a crash. Will revert the change.
I believed if app level encryption is enabled everything outgoing or incoming is encrypted...