rx.run on an empty observable confusingly errors with a CancelledError

Example code:

```python
import aioreactive as rx
async def start():
empty = await rx.run(rx.empty())
```

We get the error:

```
Exception has occurred: CancelledError
exception: no description
File "/home/gr/.pyenv/versions/3.13.2/lib/python3.13/asyncio/tasks.py", line 507, in wait_for
return await fut
...
```

If `rx.run` errors on an empty input, the error thrown should say so explicitly.

Start by reproducing the issue with the provided rx.run(rx.empty()) example, then trace how rx.run handles an empty observable and where the CancelledError originates. Done means an empty input produces an explicit, meaningful error instead of an undescribed CancelledError.

Written by the indexing model from the issue text.