add nil check for mfa enforcement config namespace on login (hashicor…

…p#20375)

* add nil check for mfa enforcement config ns

* move nil check and add changelog

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

@@ -1788,7 +1788,8 @@ 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
 		}