empty password if var not set

roles/mastodon/tasks/system_setup/prepare_database.yml

@@ -42,7 +42,7 @@
   postgresql_user:
     db: "{{ mastodon_db }}"
     name: "{{ mastodon_db_user }}"
-    password: "{{ mastodon_db_password }}"
+    password: "{{ mastodon_db_password | default('') }}"
     encrypted: yes
     login_unix_socket: "{{ mastodon_db_login_unix_socket }}"
     role_attr_flags: CREATEDB