There is no need to error out for empty <partition></partition> element
as we can just simply ignore it. This allows to simplify the function
and prepare it for new sub-elements of <resource>.
It makes the <partition> element optional so we need to reflect the
change in schema as well.
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>