`extra_template_vars` should be OK to return `None`

Got this exception and had to make sure it always returned `{}`:

```
  File ".../python3.11/site-packages/datasette/app.py", line 1049, in render_template
    assert isinstance(extra_vars, dict), "extra_vars is of type {}".format(
AssertionError: extra_vars is of type <class 'NoneType'>
```