fix: usr: Prevent aborts during expired cache dumps

Running rndc dumpdb -expired could cause named to abort when the cache contained internal deletion markers for records that had already been removed. BIND now skips those markers when preparing expired cache dumps, so the dump includes only real cached records and completes normally.

Closes #6064

Merge branch '6064-skip-nonexistent-headers' into 'main'

See merge request isc-projects/bind9!12387