Bug fix: NullableColumnAccessor.hasNext must take nulls into account

Author: liancheng

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

@@ -54,4 +54,6 @@ private[sql] trait NullableColumnAccessor extends ColumnAccessor {
 
     pos += 1
   }
+
+  abstract override def hasNext = seenNulls < nullCount || super.hasNext
 }

sql/core/src/test/scala/org/apache/spark/sql/columnar/NullableColumnAccessorSuite.scala

@@ -68,12 +68,16 @@ class NullableColumnAccessorSuite extends FunSuite {
       val row = new GenericMutableRow(1)
 
       (0 until 4).foreach { _ =>
+        assert(accessor.hasNext)
         accessor.extractTo(row, 0)
         assert(row(0) === randomRow(0))
 
+        assert(accessor.hasNext)
         accessor.extractTo(row, 0)
         assert(row.isNullAt(0))
       }
+
+      assert(!accessor.hasNext)
     }
   }
 }