backport of commit c261445 (hashicorp#20406)

Co-authored-by: davidadeleon <56207066+davidadeleon@users.noreply.github.com>

Author: hc-github-team-secure-vault-core

changelog/20375.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+core: prevent panic on login after namespace is deleted that had mfa enforcement
+```

vault/login_mfa.go

@@ -1774,7 +1774,8 @@ ECONFIG_LOOP:
 		if err != nil {
 			return nil, fmt.Errorf("failed to find the MFAEnforcementConfig namespace")
 		}
-		if eConfig == nil || (eConfigNS.ID != ns.ID && !ns.HasParent(eConfigNS)) {
+
+		if eConfig == nil || eConfigNS == nil || (eConfigNS.ID != ns.ID && !ns.HasParent(eConfigNS)) {
 			continue
 		}