add some missing null types in phpdoc

[julienbornstein]

• In ServiceBusRestProxy: fix phpdoc @return : receiveMessage() receiveQueueMessage() and receiveSubscriptionMessage() can return null.

https://github.com/Azure/azure-sdk-for-php/blob/83225de92de1a9b9325e59bd58bc143d3df505e6/src/ServiceBus/ServiceBusRestProxy.php#L226

• 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

[microsoft-cla-retired[bot]]

All CLA requirements met.