Return an empty <vCard/> element in an IQ-result when no vCard exists.

According to XEP-0054, if no vCard exists, the server MUST return a stanza
error (which SHOULD be <item-not-found/>) or an IQ-result containing an
empty <vCard/> element.

Signed-off-by: Gu Feng <flygoast@126.com>