support empty rootURL with Ember CLI >= 5.0

[jelhan]

Until Ember CLI v5 the baseURL was set to '/' if rootURL was an empty string. Since Ember CLI v5 the baseURL is not set anymore on the config.options object at all. This change causes ember-cli-live-reload to break if applications set baseURL = '' in the config/environment.js.

TypeError: Cannot read properties of undefined (reading 'replace')
    at Class.serverMiddleware (/path/to/code/node_modules/ember-cli-inject-live-reload/lib/index.js:62:38)
    at ExpressServerTask.<anonymous> (/path/to/code/node_modules/ember-cli/lib/tasks/server/express-server.js:107:24)
    at promiseMapSeries (/path/to/code/node_modules/ember-cli/node_modules/promise-map-series/index.js:7:24)
    at async ExpressServerTask.startHttpServer (/path/to/code/node_modules/ember-cli/lib/tasks/server/express-server.js:258:5)
    at async ServeTask.run (/path/to/code/node_modules/ember-cli/lib/tasks/serve.js:96:5)
    at async Class.run (/path/to/code/node_modules/ember-cli/lib/commands/serve.js:106:5)
    at async /path/to/code/node_modules/ember-cli/lib/cli/cli.js:173:32

Nullish coalescing operator should solve the issue as it does not fall back to baseURL if rootURL is an empty string.

Please find a simple reproduction here: https://github.com/jelhan/ember-app-with-relative-root-url ember serve is working fine for the last commit using Ember CLI v4.12. If you go back one commit, which uses Ember CLI v5.0 it breaks with the error mentioned above.

[jelhan]

As an alternative pattern we could fall back to '/' not only if baseURL is an empty string but always when it is falsy. This would match previous behavior of Ember CLI more closely as options.baseURL had such a fallback.

Suggested change

	let baseURL = options.liveReloadBaseUrl ?? options.rootURL ?? options.baseURL;
	// express requires absolute paths
	if (baseURL === '') {
	baseURL = '/';
	}
	let baseURL = options.liveReloadBaseUrl || options.rootURL || options.baseURL || '/';

I'm not much opinionated on either or option.