RTDB Emulator allows write / read to a DB without a namespace

If you spin up the RTDB emulator, you can do...

```
curl -XPOST -d '"Data!"' localhost:9000/test.json
```

then

```
curl -XGET localhost:9000/test.json
```

These ops succeed but do not trigger functions as they appear to be going to some invisible (default?) namespace. Maybe we should make ops without an `ns` flag point to the only know `ns` if there's only one, otherwise return an error?