[9.20] fix: usr: Fix the processing of empty catalog zone ACLs

The :iscman:`named` process could terminate unexpectedly when
processing a catalog zone ACL in an APL resource record that
was completely empty. This has been fixed.

Closes #5801

Backport of MR !11740

Merge branch 'backport-5801-catz-empty-apl-rr-bug-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11759