[ServiceBus] Fix NPE in instrumentProcess

[lmolkova]

When an error happens during receiving a session, we can return ServiceBusMessageContext with null message

azure-sdk-for-java/sdk/servicebus/azure-messaging-servicebus/src/main/java/com/azure/messaging/servicebus/ServiceBusSessionReceiver.java

Line 125 in f079ad7

return Mono.just(new ServiceBusMessageContext(getSessionId(), error));

In such case, we log an error and should not trace or record any metrics. The instrumentation code should back off if the message is null.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-messaging-servicebus

[anuchandy]

thank you! (just remember to fixup the todo in changelog)