CCMA: App Assessment UI won't load properly if the admin user has an invalid or NULL email address

If the Confluence Admin user running the migration in CCMA has an invalid (or empty/NULL) email address, the App Assessment UI in CCMA won't load properly and an error pop-up will surface.

By "load properly", this means the user won't have the Assess your apps/Prepare your apps menus nor the View app assessment button available.

Other parts of the Assess interface appear to work normally.

Steps to Reproduce

1. Log in to Confluence Server/DC with an Admin User that does not have a valid email address
2. Try to create a plan in the Confluence Cloud Migration Assistant App

Expected Results

CCMA's App Assessment UI loads properly and this invalid email problem is caught in the Assess and prepare your users section.

Actual Results

CCMA's home page will not allow the App Assessment area to load properly and an error pop up will surface on the left bottom portion of the UI, without any indication of the actual problem.

Log errors

The below exception is thrown in the Confluence Application log:

2024-02-02 18:07:21,135 ERROR [http-nio-10.254.0.5-8090-exec-9] [common.error.jersey.ThrowableExceptionMapper] toResponse Uncaught exception 83c498ea-c928-4210-8973-d8dce3eba3ac thrown by REST service: CacheLoader returned null for key .
 -- url: /rest/migration/latest/app | userName: admin.username | referer: confluencesite.com/admin/migration.action | traceId: 3d227169e40286d8
com.google.common.cache.CacheLoader$InvalidCacheLoadException: CacheLoader returned null for key . 

Workaround

Add an email address for the admin user:

While the error is not caught in the Assess and prepare your users section, the solution for missing email problems found in that section will resolve the problem.  Please see Resolve missing email addresses when migrating for more details.