fix: guard against corrupted user creds

[manekinekko]

Pull Request Template

Description

Somehow the underlying SDK was keeping a copy of the cache from previous auths processes. However, that cache was corrupted and some creds were undefined (username and tokenAudience).

See https://github.com/AzureAD/azure-activedirectory-library-for-nodejs/issues/215

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

[shmool]

Thanks!