Currently the spec defines the messages field in the JSON body of requests to PUT /_matrix/client/r0/sendToDevice/{eventType}/{txnId} as not required. It's not really clear what the homeserver should do when messages is not provided.

Indeed, there isn't anything for the homeserver to do when messages is not provided, and thus not including message is a bit pointless.

I feel that we should mark messages as a required field. It doesn't really make sense to define a default message.

Note that Synapse currently Internal Server Errors when messages is not provided, though this is fixed by matrix-org/synapse#8975.