Avoid warning about uninitialized ivar

Co-authored-by: Farid Zakaria <farid.m.zakaria@gmail.com>

Author: pitr-ch

lib/concurrent-ruby/concurrent/thread_safe/util/data_structures.rb

@@ -42,7 +42,7 @@ def self.make_synchronized_on_rbx(klass)
           private
 
           def _mon_initialize
-            @_monitor = Monitor.new unless @_monitor # avoid double initialisation
+            @_monitor ||= Monitor.new # avoid double initialisation
           end
 
           def self.new(*args)