[SPARK-23960][SQL][MINOR] Mark HashAggregateExec.bufVars as transient

## What changes were proposed in this pull request?

Mark `HashAggregateExec.bufVars` as transient to avoid it from being serialized.
Also manually null out this field at the end of `doProduceWithoutKeys()` to shorten its lifecycle, because it'll no longer be used after that.

## How was this patch tested?

Existing tests.

Author: Kris Mok <kris.mok@databricks.com>

Closes #21039 from rednaxelafx/codegen-improve.

Author: rednaxelafx

sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/HashAggregateExec.scala

@@ -174,8 +174,8 @@ case class HashAggregateExec(
     }
   }
 
-  // The variables used as aggregation buffer. Only used for aggregation without keys.
-  private var bufVars: Seq[ExprCode] = _
+  // The variables used as aggregation buffer. Only used in codegen for aggregation without keys.
+  @transient private var bufVars: Seq[ExprCode] = _
 
   private def doProduceWithoutKeys(ctx: CodegenContext): String = {
     val initAgg = ctx.addMutableState(CodeGenerator.JAVA_BOOLEAN, "initAgg")
@@ -238,6 +238,8 @@ case class HashAggregateExec(
          | }
        """.stripMargin)
 
+    bufVars = null  // explicitly null this field out to allow the referent to be GC'd sooner
+
     val numOutput = metricTerm(ctx, "numOutputRows")
     val aggTime = metricTerm(ctx, "aggTime")
     val beforeAgg = ctx.freshName("beforeAgg")