Allow accepting blank session (Azure#24737)

Author: JoshLove-msft

sdk/servicebus/Azure.Messaging.ServiceBus/src/Amqp/AmqpReceiver.cs

@@ -220,7 +220,7 @@ private async Task<ReceivingAmqpLink> OpenReceiverLinkAsync(
                         throw new ServiceBusException(true, Resources.SessionFilterMissing);
                     }
 
-                    if (string.IsNullOrWhiteSpace(tempSessionId))
+                    if (tempSessionId == null)
                     {
                         link.Session.SafeClose();
                         throw new ServiceBusException(true, Resources.AmqpFieldSessionId);

sdk/servicebus/Azure.Messaging.ServiceBus/tests/Client/ServiceBusClientLiveTests.cs

@@ -220,5 +220,16 @@ public async Task AcceptNextSessionRespectsCancellation()
                 Assert.Less(stop - start, duration.Add(duration));
             }
         }
+
+        [Test]
+        public async Task CanAcceptBlankSession()
+        {
+            await using (var scope = await ServiceBusScope.CreateWithQueue(enablePartitioning: false, enableSession: true))
+            {
+                var client = CreateClient();
+                var receiver = await client.AcceptSessionAsync(scope.QueueName, "");
+                Assert.AreEqual("", receiver.SessionId);
+            }
+        }
     }
 }