fix(intercom): safely check for app_id in optional param

[yharaskrik]

Summary of changes:
Because intercomData is optional in the boot method then we need to use optional chaining to check for app_id otherwise it will error if that parameter is not passed on

Intended/example use case:
app_id is configured in the forRoot method so when calling .boot() without a paramter the line const app_id = intercomData.app_id ? intercomData.app_id : this.config.appId will error since intercomData is optional.

Checklist:

• npm run build runs without error
• ng serve spawns app, Intercom messenger is visible and interactive, and there are no errors in the console

Closes issue: #