fix: dev: Allow negative RRSIGs in the qpcache again

The previous refactoring added an assertion failure when negative RRSIG
would be added to the cache database.  As result, any query for RRSIG in
any unsigned zone would trigger that assertion failure.

Allow the negative RRSIG entries to be stored in the cache database
again as not caching these would trigger new remote fetch every time
such query would be received from a client.

Closes #5489

Merge branch '5489-allow-negative-RRSIGs-in-qpcache' into 'main'

See merge request isc-projects/bind9!10876