sftp: do not show creation time when we do not know it
Summary:
creation time (aka birth time) is file system dependent, if the fs doesn't
support it we seem to get createtime==0. skip over it when that is the
case.
BUG: 375305
FIXED-IN: 20.04
Test Plan: browsing a directory on btrfs over sftp doesn't show creation time as 1970
Reviewers: ngraham, bruns, meven
Reviewed By: ngraham, bruns, meven
Subscribers: meven, bruns, kde-frameworks-devel, kfm-devel
Tags: Dolphin, Frameworks
Differential Revision: https://phabricator.kde.org/D27873