fix: a not initialized instance variable warned on Ruby 2.7 (#347)

Author: supercaracal

lib/redis_client/cluster/node.rb

@@ -106,6 +106,7 @@ def initialize(
         @topology = klass.new(pool, @concurrent_worker, **kwargs)
         @config = config
         @mutex = Mutex.new
+        @last_reloaded_at = nil
       end
 
       def inspect