Add test for client middleware short circuit

lib/sidekiq/client.rb

@@ -49,7 +49,7 @@ def self.push(item)
             conn.rpush("queue:#{normed['queue']}", payload)
           end
         end
-      end
+      end if normed
       pushed ? normed['jid'] : nil
     end
 

test/test_client.rb

@@ -117,6 +117,26 @@ class BWorker < BaseWorker
     sidekiq_options 'retry' => 'b'
   end
 
+  describe 'client middleware' do
+
+    class Stopper
+      def call(worker_class, message, queue)
+        yield if message['args'].first.odd?
+      end
+    end
+
+    it 'can stop some of the jobs from pushing' do
+      Sidekiq.client_middleware.add Stopper
+      begin
+        assert_equal nil, Sidekiq::Client.push('class' => MyWorker, 'args' => [0])
+        assert_match /[0-9a-f]{12}/, Sidekiq::Client.push('class' => MyWorker, 'args' => [1])
+        assert_equal 1, Sidekiq::Client.push_batch('class' => MyWorker, 'args' => [[0], [1]])
+      ensure
+        Sidekiq.client_middleware.remove Stopper
+      end
+    end
+  end
+
   describe 'inheritance' do
     it 'should inherit sidekiq options' do
       assert_equal 'base', AWorker.get_sidekiq_options['retry']