[SPARK-48218][CORE] TransportClientFactory.createClient may NPE cause…

… FetchFailedException

### What changes were proposed in this pull request?
This PR aims to add a check for `TransportChannelHandler` to be non-null in the `TransportClientFactory.createClient` method.

### Why are the changes needed?

Line 178 synchronized (handler) , handler == null

org.apache.spark.network.client.TransportClientFactory#createClient(java.lang.String, int, boolean)
```java
      TransportChannelHandler handler = cachedClient.getChannel().pipeline()
        .get(TransportChannelHandler.class);
      synchronized (handler) {
        handler.getResponseHandler().updateTimeOfLastRequest();
      }
```

```java
org.apache.spark.shuffle.FetchFailedException
	at org.apache.spark.storage.ShuffleBlockFetcherIterator.throwFetchFailedException(ShuffleBlockFetcherIterator.scala:1180)
	at org.apache.spark.storage.ShuffleBlockFetcherIterator.next(ShuffleBlockFetcherIterator.scala:913)
	at org.apache.spark.storage.ShuffleBlockFetcherIterator.next(ShuffleBlockFetcherIterator.scala:84)
	at org.apache.spark.util.CompletionIterator.next(CompletionIterator.scala:29)

Caused by: java.lang.NullPointerException
	at org.apache.spark.network.client.TransportClientFactory.createClient(TransportClientFactory.java:178)
	at org.apache.spark.network.shuffle.ExternalBlockStoreClient.lambda$fetchBlocks$0(ExternalBlockStoreClient.java:128)
	at org.apache.spark.network.shuffle.RetryingBlockTransferor.transferAllOutstanding(RetryingBlockTransferor.java:154)
	at org.apache.spark.network.shuffle.RetryingBlockTransferor.start(RetryingBlockTransferor.java:133)
	at org.apache.spark.network.shuffle.ExternalBlockStoreClient.fetchBlocks(ExternalBlockStoreClient.java:139)
```

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #46506 from cxzl25/SPARK-48218.

Authored-by: sychen <sychen@ctrip.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

common/network-common/src/main/java/org/apache/spark/network/client/TransportClientFactory.java

@@ -171,8 +171,10 @@ public TransportClient createClient(String remoteHost, int remotePort, boolean f
       // this code was able to update things.
       TransportChannelHandler handler = cachedClient.getChannel().pipeline()
         .get(TransportChannelHandler.class);
-      synchronized (handler) {
-        handler.getResponseHandler().updateTimeOfLastRequest();
+      if (handler != null) {
+        synchronized (handler) {
+          handler.getResponseHandler().updateTimeOfLastRequest();
+        }
       }
 
       if (cachedClient.isActive()) {