common/entity_name: do not populate type_id string for type==0

common/entity_name: do not populate type_id string for type==0

The EntityName ctor leaves type == 0.  If we encode and
then decode that value, we end up calling set(), which
tries (and fails) to populate the type_id string, leaving
you with an instance that renders as 'unknown.' instead
of ''.

Signed-off-by: Sage Weil <sage@redhat.com>