[Issue] Front-end Templates don't work if App Insights is not properly configured

[brunoborges]

See https://github.com/Azure/azure-dev/blob/main/templates/todo/web/react-fluent/src/services/telemetryService.ts#L29-L30.

If connection string is not set, the loadApplicationInsights triggers an exception that is not handled, and the front-end fails to load.

This mainly blocks developer from running the web app locally before provisioning Azure resources.