Avoid infinite waiting for consumer close (apache#11347)

### Motivation
If there are two Events
- EventA: Server close the consumer
- EventB: consumer close self

If EventA and EventB concurrently, then the `cnx.ctx()` has probability to be null. Which occurs a NPE, lead to infinite wait future

### Modifications

If the `cnx.ctx()` is already null, we ignore the exception too.

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerImpl.java

@@ -28,6 +28,7 @@
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.Unpooled;
+import io.netty.channel.ChannelHandlerContext;
 import io.netty.util.Recycler;
 import io.netty.util.Recycler.Handle;
 import io.netty.util.ReferenceCountUtil;
@@ -926,7 +927,8 @@ public CompletableFuture<Void> closeAsync() {
         } else {
             ByteBuf cmd = Commands.newCloseConsumer(consumerId, requestId);
             cnx.sendRequestWithId(cmd, requestId).handle((v, exception) -> {
-                boolean ignoreException = !cnx.ctx().channel().isActive();
+                final ChannelHandlerContext ctx = cnx.ctx();
+                boolean ignoreException = ctx == null || !ctx.channel().isActive();
                 if (ignoreException && exception != null) {
                     log.debug("Exception ignored in closing consumer", exception);
                 }