DNS AM: add a bit of defensive programming (#4092) #6710

Job	Run time
Type hints check	34s
Code health check	1m 13s
Build doc	6m 14s
Check SPDX identifiers	8s
pyca/cryptography test	26s
CodeQL analysis	3m 35s
ubuntu-latest 3.7 non_root -K scanner	3m 59s
ubuntu-latest 3.8 non_root -K scanner	3m 31s
ubuntu-latest 3.9 non_root -K scanner	3m 40s
ubuntu-latest 3.10 non_root -K scanner	3m 52s
ubuntu-latest 3.11 non_root -K scanner	3m 23s
ubuntu-latest 3.12 non_root -K scanner	4m 11s
ubuntu-latest 3.12 root -K scanner	5m 34s
ubuntu-latest pypy3.9 root -K scanner	4m 59s
ubuntu-latest libpcap 3.12 root -K scanner	4m 50s
macos-12 3.12 both -K scanner	6m 26s
ubuntu-latest 3.12 root -k scanner	4m 1s
ubuntu-latest pypy3.9 root -k scanner	3m 20s
macos-12 3.12 both -k scanner	2m 12s
	1h 6m 8s

Provide feedback