Fix error on entity_config missing (#11561)

If the `google_assistant` key exists in the config but has no
`entity_config` key under it you'll get an error.

```
  File "/Users/pkates/src/home-assistant/homeassistant/components/google_assistant/http.py", line 51, in is_exposed
    entity_config.get(entity.entity_id, {}).get(CONF_EXPOSE)
AttributeError: 'NoneType' object has no attribute 'get'
```