SPARK-54449: Check if off heap config is enabled before getting off heap size value

Author: VindhyaG

core/src/main/scala/org/apache/spark/memory/MemoryManager.scala

@@ -58,7 +58,7 @@ private[spark] abstract class MemoryManager(
   onHeapStorageMemoryPool.incrementPoolSize(onHeapStorageMemory)
   onHeapExecutionMemoryPool.incrementPoolSize(onHeapExecutionMemory)
 
-  protected[this] val maxOffHeapMemory = conf.get(MEMORY_OFFHEAP_SIZE)
+  protected[this] val maxOffHeapMemory = conf.get(MEMORY_OFFHEAP_ENABLED)? conf.get(MEMORY_OFFHEAP_SIZE) : 0
   protected[this] val offHeapStorageMemory =
     (maxOffHeapMemory * conf.get(MEMORY_STORAGE_FRACTION)).toLong