"rndc signing -nsec3param ..." requests are silently ignored for zones which are not loaded

If rndc signing -nsec3param ... is ran for a zone which has not yet been loaded or transferred (i.e. its db field is NULL), it will be silently ignored by named despite rndc logging an "nsec3param request queued" message, which is misleading.

I believe this issue also contributes to a certain failure mode of the "inline" system test (see #285 (closed)) being triggered.