modules/chanserv/clone.c: Gracefully handle chanacs 'setter' being NULL

Using ChanServ CLONE, where some chanacs entries in services.db were created in a legacy format, generates assert spam.

```
PRIVMSG ChanServ :REGISTER #TestChannel
PRIVMSG ChanServ :CLONE #LegacyChannel #TestChannel
<OperServ> (entity.c:84 myentity_find): warning: assertion 'name != ((void *)0)' failed.
<OperServ> (entity.c:84 myentity_find): warning: assertion 'name != ((void *)0)' failed.
<OperServ> (entity.c:84 myentity_find): warning: assertion 'name != ((void *)0)' failed.
etc...
```