getAll now returns an empty object when there are no resources

Before you had to handle `undefined` yourself.

Refs #597
getListing returns an empty object when no items are found; update CHANGELOG
Since this PR changes this behavior for getAll, it makes sense to
do the same for getListing at the same time.
This commits also adds both changes to the CHANGELOG.
Adapt test for getListing returning empty object on 404
Merge pull request #674 from remotestorage/feature/597-getAll_returns_empty_object

getAll now returns an empty object when there are no resources (continued from #671)