Starting Ember 5.8.x with a blank `rootURL` throws an error because `baseURL` is `undefined` and so has no `replace` method

We're upgrading from Ember 3.28.x to 5.8.x, and have found an issue when starting our Ember server.

We've got no `liveReloadBaseUrl` config and no `baseURL` config, but we do have an empty string for our `rootURL` config.

The problem is that https://github.com/ember-cli/ember-cli-inject-live-reload/blob/master/lib/index.js#L51 checks as follows:
```javascript
let baseURL = options.liveReloadBaseUrl || options.rootURL || options.baseURL;
...
let baseURLWithoutHost = baseURL.replace(/^https?:\/\/[^/]+/, '');
```

This is problematic because with an `undefined` value for both `options.liveReloadBaseUrl` and `options.baseURL`, `baseURL` resolves to `undefined` and so the `.replace` call is failing, even though we have a defined value of `''` for `options.rootURL`.

I've added a temporary workaround (setting `baseURL` to `''` in `.ember-cli`), and that appears to work... however, it would be nice to not have to add this workaround.

Would it be possible to use `??` instead of `||` for the falsey check?