fix: handle None from lookup('file') when postfix-accounts.cf doesn't exist

lookup(..., errors='ignore') returns None (not empty string) for missing files.
Use | default('', true) to also convert falsy None to empty string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

roles/tools/tasks/main.yml

@@ -145,7 +145,7 @@
     - { address: "jack@{{ domain_base }}",    password: "{{ mailserver_jack_password }}" }
   when: >
     not postfix_accounts_file.stat.exists or
-    item.address not in (lookup('file', tools_root + '/mailserver/config/postfix-accounts.cf', errors='ignore') | default(''))
+    item.address not in (lookup('file', tools_root + '/mailserver/config/postfix-accounts.cf', errors='ignore') | default('', true))
 
 # ── DKIM ─────────────────────────────────────────────────────────────────────
 - name: Check if DKIM key exists