Disable IMAP auth check in Rake task when Microsoft Graph is configured

MailRoom supports two inbox modes: IMAP and Microsoft Graph. The former uses the IMAP protocol, and the latter uses a REST API to retrieve e-mail.

When we added support for Microsoft Graph, we did not account for this in the gitlab:incoming_email:check Rake task.

For now, disable the IMAP check if is it not configured. We may want to consider adding a Microsoft Graph-specific check, but MailRoom will exit if the Microsoft Graph credentials are not configured properly.

Relates to #214900 (comment 656345704)