SMTP configuration should allow empty credentials

Problem: it's not possible to specify empty credentials, because they are automatically set and cannot be removed.
As a result, we're unable to send emails.

Scenario:
One of our customers doesn't need any credentials and not being able to unset them leads to this issue: STASH-3482

However, if we try to send a test email with empty credentials (as mentioned in this comment related to Jira: JRA-27623) that test email is correctly sent.