message: fatal crash on invalid message format

An invalid `cylc message` can take out the scheduler.

(note the message would have to be authenticated for this to happen)

### Reproducible Example

```cylc
[scheduling]
    [[graph]]
        R1 = foo

[runtime]
    [[foo]]
        script = cylc message "$CYLC_WORKFLOW_ID" -- a b c
```

```
$ cylc vip -N .
...
ERROR - Workflow shutting down - WorkflowConfigError: Illegal task name: None
```