Customer cannot create a OneDrive connector.

When a customer's Azure AD tenant contains users with a null userPrincipalName (e.g. guest/external users, incomplete
hybrid AD sync, or service accounts), the OneDrive connector health check fails with a DecodingException during
deserialization of the Microsoft Graph API /users response. This prevents the connector from being saved/updated,
leaving it in an "Action Required" or pending deletion state.

Root Cause

MicrosoftUser.userPrincipalName is declared as a non-nullable String. When
Microsoft Graph API returns a user object with userPrincipalName: null, causing serialization to fail and crashing the process.