ARTEMIS-1389 Add non-null check to ServerSessionImpl.doClose()

An Openwire connection creates an internal session used to track
transaction status, it doesn't have a session callback. When
the connection is closed, the core session should check if
callback is null to avoid NPE.

artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/ServerSessionImpl.java

@@ -348,7 +348,9 @@ public class ServerSessionImpl implements ServerSession, FailureListener {
    }
 
    protected void doClose(final boolean failed) throws Exception {
-      callback.close(failed);
+      if (callback != null) {
+         callback.close(failed);
+      }
       synchronized (this) {
          if (!closed) {
             server.callBrokerPlugins(server.hasBrokerPlugins() ? plugin -> plugin.beforeCloseSession(this, failed) : null);