[SPARK-11006] Rename NullColumnAccess as NullColumnAccessor

davies
I think NullColumnAccessor follows same convention for other accessors

Author: tedyu <yuzhihong@gmail.com>

Closes #9028 from tedyu/master.

sql/core/src/main/scala/org/apache/spark/sql/columnar/ColumnAccessor.scala

@@ -62,7 +62,7 @@ private[sql] abstract class BasicColumnAccessor[JvmType](
   protected def underlyingBuffer = buffer
 }
 
-private[sql] class NullColumnAccess(buffer: ByteBuffer)
+private[sql] class NullColumnAccessor(buffer: ByteBuffer)
   extends BasicColumnAccessor[Any](buffer, NULL)
   with NullableColumnAccessor
 
@@ -125,7 +125,7 @@ private[sql] object ColumnAccessor {
     val buf = buffer.order(ByteOrder.nativeOrder)
 
     dataType match {
-      case NullType => new NullColumnAccess(buf)
+      case NullType => new NullColumnAccessor(buf)
       case BooleanType => new BooleanColumnAccessor(buf)
       case ByteType => new ByteColumnAccessor(buf)
       case ShortType => new ShortColumnAccessor(buf)