[3.1.6 Backport] UnmarshalDocumentSyncDataFromFeed doesn't return any present user xattrs if sync data doesn't exist on the doc

UnmarshalDocumentSyncDataFromFeed doesn't seem to return any user xattrs defined on a doc if the doc doesn't have any sync xattr defined.

Repro steps:

1. Have a db setup with a sync function assigning a channel defined in your defined user xattr on the database
2. Add doc through SDK with user xattr defined
3. Wait for import feed to pick up the doc

Example sync function to use:

function(doc, oldDoc, meta) {

    if (meta.xattrs.channels === undefined) {

        console.log("no user_xattr_key defined");

        throw ({

            forbidden: "Missing required property - metadata channel info"

        });

    } else {

        console.log(meta.xattrs.channels);

        channel(meta.xattrs.channels);

    }

}