Fixed get_current_task() crashing with no current task on asyncio

[AmirF194]

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Changes

Fixes #773.

run_sync_from_thread() schedules its callback via loop.call_soon_threadsafe(), which runs outside of any asyncio.Task, so current_task() is None in there. get_current_task() builds AsyncIOTaskInfo(current_task()) regardless, and AsyncIOTaskInfo.__init__ does _task_states.get(task) on a WeakKeyDictionary, which can't weak-reference None and blows up with the opaque TypeError from the issue.

Same "no current task" case #1163 already guards for CancelScope.__enter__ and cancel_shielded_checkpoint(), just a call site it doesn't touch. This raises a clear RuntimeError instead, matching that shape.

Checklist

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- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

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