Refresh collections individually if no CTag found
This patch is necessary to get CardDAV with Google to work.
For some reason they do not return a ctag for the home set
and we have to fetch the individual collections instead.
This patch solves this by simply adding a fallback for when
the ctag is empty.
Summary: Fixes T9490
Reviewed By: cmollekopf
Maniphest Tasks: T9490
Differential Revision: https://phabricator.kde.org/D15055