WebSocketRegistryListener handles null Session in SessionDisconnectEvent

Fixes gh-77

spring-session/src/test/java/org/springframework/session/web/socket/handler/WebSocketRegistryListenerTests.java

@@ -116,6 +116,16 @@ public class WebSocketRegistryListenerTests {
         verify(wsSession,times(0)).close(any(CloseStatus.class));
     }
 
+    @Test
+    public void onApplicationEventConnectDisconnectNullSession() throws Exception {
+        listener.onApplicationEvent(connect);
+        attributes.clear();
+
+        listener.onApplicationEvent(disconnect);
+
+        // no exception
+    }
+
     @Test
     public void onApplicationEventConnectConnectDisonnect() throws Exception {
         listener.onApplicationEvent(connect);