[SPARK-23047][PYTHON][SQL] Change MapVector to NullableMapVector in ArrowColumnVector

## What changes were proposed in this pull request?
This PR changes usage of `MapVector` in Spark codebase to use `NullableMapVector`.

`MapVector` is an internal Arrow class that is not supposed to be used directly. We should use `NullableMapVector` instead.

## How was this patch tested?

Existing test.

Author: Li Jin <ice.xelloss@gmail.com>

Closes #20239 from icexelloss/arrow-map-vector.

(cherry picked from commit 4e6f8fb)
Signed-off-by: hyukjinkwon <gurwls223@gmail.com>

Author: icexelloss

sql/core/src/main/java/org/apache/spark/sql/vectorized/ArrowColumnVector.java

@@ -247,8 +247,8 @@ public ArrowColumnVector(ValueVector vector) {
 
       childColumns = new ArrowColumnVector[1];
       childColumns[0] = new ArrowColumnVector(listVector.getDataVector());
-    } else if (vector instanceof MapVector) {
-      MapVector mapVector = (MapVector) vector;
+    } else if (vector instanceof NullableMapVector) {
+      NullableMapVector mapVector = (NullableMapVector) vector;
       accessor = new StructAccessor(mapVector);
 
       childColumns = new ArrowColumnVector[mapVector.size()];
@@ -553,9 +553,16 @@ final int getArrayOffset(int rowId) {
     }
   }
 
+  /**
+   * Any call to "get" method will throw UnsupportedOperationException.
+   *
+   * Access struct values in a ArrowColumnVector doesn't use this accessor. Instead, it uses getStruct() method defined
+   * in the parent class. Any call to "get" method in this class is a bug in the code.
+   *
+   */
   private static class StructAccessor extends ArrowVectorAccessor {
 
-    StructAccessor(MapVector vector) {
+    StructAccessor(NullableMapVector vector) {
       super(vector);
     }
   }

sql/core/src/test/scala/org/apache/spark/sql/execution/vectorized/ArrowColumnVectorSuite.scala

@@ -322,6 +322,42 @@ class ArrowColumnVectorSuite extends SparkFunSuite {
     allocator.close()
   }
 
+  test("non nullable struct") {
+    val allocator = ArrowUtils.rootAllocator.newChildAllocator("struct", 0, Long.MaxValue)
+    val schema = new StructType().add("int", IntegerType).add("long", LongType)
+    val vector = ArrowUtils.toArrowField("struct", schema, nullable = false, null)
+      .createVector(allocator).asInstanceOf[NullableMapVector]
+
+    vector.allocateNew()
+    val intVector = vector.getChildByOrdinal(0).asInstanceOf[IntVector]
+    val longVector = vector.getChildByOrdinal(1).asInstanceOf[BigIntVector]
+
+    vector.setIndexDefined(0)
+    intVector.setSafe(0, 1)
+    longVector.setSafe(0, 1L)
+
+    vector.setIndexDefined(1)
+    intVector.setSafe(1, 2)
+    longVector.setNull(1)
+
+    vector.setValueCount(2)
+
+    val columnVector = new ArrowColumnVector(vector)
+    assert(columnVector.dataType === schema)
+    assert(columnVector.numNulls === 0)
+
+    val row0 = columnVector.getStruct(0, 2)
+    assert(row0.getInt(0) === 1)
+    assert(row0.getLong(1) === 1L)
+
+    val row1 = columnVector.getStruct(1, 2)
+    assert(row1.getInt(0) === 2)
+    assert(row1.isNullAt(1))
+
+    columnVector.close()
+    allocator.close()
+  }
+
   test("struct") {
     val allocator = ArrowUtils.rootAllocator.newChildAllocator("struct", 0, Long.MaxValue)
     val schema = new StructType().add("int", IntegerType).add("long", LongType)