Set default for mailman_rest_password

Allows account-automation-common role to succeed for non-mail
test setups.

roles/account-automation-common/vars/main.yml

@@ -15,7 +15,7 @@ account_automation_config:
   frontend_ip_addrs: "{{ services['frontend'].hosts | map('extract', hostvars) | map(attribute='ips') | flatten | sort | list }}"
   static_mail_domains: "{{ static_mail_domains | sort | list }}"
   mailman3_admin_user: "admin"
-  mailman3_admin_password: "{{ mailman_rest_password }}"
+  mailman3_admin_password: "{{ mailman_rest_password | default('') }}"
   mailer_params:
     gpg_key_id: "{{ account_automation_gpg_key_id | default('') }}"
     gpg_homedir: "/etc/ai/gpg"