[9.20] fix: usr: Fix a bug in dnssec-signzone related to keys being offline

In the case when `dnssec-signzone` is called on an already signed zone, and the private key file is unavailable, a signature that needs to be refreshed may be dropped without being able to generate a replacement. This has been fixed.

Closes #5126

Backport of MR !9951

Merge branch 'backport-5126-dnssec-signzone-retain-rrsig-if-key-is-offline-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9982