Prior to this change, EMQX default ACL has a deny rule to reject
subscribing to `#`.
For completeness, the default ACL should also deny `+/#` because
they are essentially equivalent.
when client_attrs_init expression renders to empty string,
do not initialize the attribute.
also fixed an ACL error: a template render failure for a topic
would stop the ACL checks for the following topics if more
than one topic is configured.
zmstone