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.

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