_tp_contacts_to_handles: check we pass at least one contact
_tp_contacts_to_handles: check we pass at least one contact

All the functions using this were already documenting that we should pass at
least one contact.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib