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.
For backward compatibility, the unknown vars used in mountpoint
is kept unchanged.
e.g. '${unknown}/foo/bar' should be rendered as '${unknown}/foo/bar'
but not 'undefined/foo/bar'
Move authz result logging to common place.
Prior to this change, the final result is not logged when
fallback to the default authorization.no_match config value.
Aso, if the result is provided by a hook callback,
it's also not logged.
After this change, only the final result is logged.
The authz chain resutls can be traced (or logged at debug level).
Even though most of the time these modules will be used by
connectors, there are exceptions (namely, `emqx_rule_engine`).
Besides, they are general enough to land there, more so given
that `emqx_placeholder` is already there.
Treat "${$}" as literal "$". This allows to template express
strings, for example, of the form "${some_var_value}" where
`some_var_value` is interpolated from bindings.
This slightly changes semantics: now the attempt to create authenticator
with illegal bindings in templates will fail, instead of treating them
as literals. The runtime behaviour on the other hand should be the same.
