FIX: Ruby 2 backward compatible plugin logout redirect (#19845)

This is a very subtle one. Setting the redirect URL is done by passing
a hash through a Discourse event. This is broken on Ruby 2 since the
support for keyword arguments in events was added.

In Ruby 2 the last argument is cast to keyword arguments if it is a
hash. The key point here is that creates a new copy of the hash, so
what the plugin is modifying is not the hash that was passed.

app/controllers/session_controller.rb

@@ -594,7 +594,7 @@ class SessionController < ApplicationController
       client_ip: request&.ip,
       user_agent: request&.user_agent,
     }
-    DiscourseEvent.trigger(:before_session_destroy, event_data)
+    DiscourseEvent.trigger(:before_session_destroy, event_data, **Discourse::Utils::EMPTY_KEYWORDS)
     redirect_url = event_data[:redirect_url]
 
     reset_session

lib/discourse.rb

@@ -12,6 +12,9 @@ module Discourse
   class Utils
     URI_REGEXP ||= URI.regexp(%w[http https])
 
+    # TODO: Remove this once we drop support for Ruby 2.
+    EMPTY_KEYWORDS ||= {}
+
     # Usage:
     #   Discourse::Utils.execute_command("pwd", chdir: 'mydirectory')
     # or with a block