add some missing null types in phpdoc

- In ServiceBusRestProxy/IServiceBus, fix phpdoc @return : receiveMessage() receiveQueueMessage() and receiveSubscriptionMessage() can return null
- In BrokerProperties, by default, when initiate an instance of the class, all properties are unset, so they are null. Fixing Phpdoc for properties, getters and setters
- In BrokeredMessage, also some phpdoc type fixes

Author: julienbornstein

src/ServiceBus/Internal/IServiceBus.php

@@ -88,7 +88,7 @@ public function sendQueueMessage($queueName, BrokeredMessage $brokeredMessage);
      *
      * @throws Exception
      *
-     * @return BrokeredMessage
+     * @return BrokeredMessage|null
      */
     public function receiveQueueMessage($queueName, ReceiveMessageOptions $receivedMessageOptions = null);
 
@@ -102,7 +102,7 @@ public function receiveQueueMessage($queueName, ReceiveMessageOptions $receivedM
      *
      * @throws Exception
      *
-     * @return BrokeredMessage
+     * @return BrokeredMessage|null
      */
     public function receiveMessage($path, ReceiveMessageOptions $receiveMessageOptions);
 
@@ -128,7 +128,7 @@ public function sendTopicMessage($topicName, BrokeredMessage $brokeredMessage);
      *
      * @throws Exception
      *
-     * @return BrokeredMessage
+     * @return BrokeredMessage|null
      */
     public function receiveSubscriptionMessage(
         $topicName,