Fix a potential NPE error in the sized hash join (#12101)

This PR fixes a potential NPE error in the sized hash join by checking the buffer's existence before actually closing it.
Otherwise duplicate call to this close method will lead to a NPE error.

It is a quite simple change so no tests for it.

sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuShuffledSizedHashJoinExec.scala

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2024, NVIDIA CORPORATION.
+ * Copyright (c) 2024-2025, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1105,7 +1105,7 @@ sealed trait SpillableHostConcatResult extends AutoCloseable {
     SpillableHostBuffer(hmb, hmb.getLength, SpillPriorities.ACTIVE_BATCHING_PRIORITY)
   }
 
-  override def close(): Unit = {
+  override def close(): Unit = if (buffer != null) {
     buffer.close()
     buffer = null
   }