data store: blank task state

[oliver-sanders]

A user has encountered a traceback in cylc tui which suggests that a task had a blank status:

  File "/.../cylc-8.3.6-1/lib/python3.9/site-packages/cylc/flow/tui/util.py", line 118, in get_task_icon
    ret.append(TASK_ICONS[status])
KeyError: ''

Tasks in the store should always have a status, even far future tasks should be waiting.

It's not clear how to reproduce this traceback (I haven't seen it before), possibly just some awkward edge case 🤦. We could protect Tui against this with a fallback to waiting, but it would be much better to find and cure the issue than cover it over.