IMAP resource: do not prompt for password with GSSAPI auth

Summary:
GSSAPI (e.g. when used with Kerberos) does not use passwords, so make sure the user is not prompted unnecessarily.

BUG: 383279
FIXED-IN: 19.12.0

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D25499