Handle nil logger, fixes #4240

Changes.md

@@ -2,6 +2,11 @@
 
 [Sidekiq Changes](https://github.com/mperham/sidekiq/blob/master/Changes.md) | [Sidekiq Pro Changes](https://github.com/mperham/sidekiq/blob/master/Pro-Changes.md) | [Sidekiq Enterprise Changes](https://github.com/mperham/sidekiq/blob/master/Ent-Changes.md)
 
+HEAD
+---------
+
+- Gracefully handle `Sidekiq.logger = nil` [#4240]
+
 6.0
 ---------
 

lib/sidekiq.rb

@@ -199,6 +199,10 @@ module Sidekiq
   end
 
   def self.logger=(logger)
+    if logger.nil?
+      self.logger.level = Logger::FATAL
+      return self.logger
+    end
     @logger = logger
   end