do not substitute when content-id is empty

When Content-ID is just "<>" the result is really strange:

```
--=_mixed 0123456789012345_=
Content-Transfer-Encoding: quoted-printable
CContent-ID: <>

Link: <a href="http://www.invalid">http://www.invalid</a>
```

is shown as 

Links: <a href="/otrs/index.pl?Action=AgentTicketAttachment;....."http://www.invalid"/otrs/index.pl?Action=AgentTicketAttachment;....." target="/otrs/index.pl?Action=AgentTicketAttachment;....."_blank"/otrs/index.pl?Action=AgentTicketAttachment;.....">http://www.invalid</a>