persistentDict: Do not warn about missing checksum when metadata is empty

When parsing the storage metadata, when none is present, accept the metadata
as valid and log that there's no metadata. In this case  do not log a warning
informing no checksum is found as there is no metadata.

Change-Id: Ic6627fef453a62603d93bfb8aee66ebedcbbf190
Bug-Url: https://bugzilla.redhat.com/1073989
Signed-off-by: Xavi Francisco <xfrancis@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27528
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>