Don't let Datasette(path) without a list cause weird errors

I got a confusing `sqlite3.OperationalError: disk I/O error` error in my tests, it turned out it was because this:
```python
ds = Datasette(path)
```
Should have been this:
```python
ds = Datasette([path])
```

_Originally posted by @simonw in https://github.com/simonw/datasette-faiss/issues/1#issuecomment-1378252673_
      