private static void verifyNulls(ColumnBuffer buffer, int shift) {
    BitSet nulls = buffer.getNulls();
    for (int i = 0; i < buffer.size(); i++) {
      Assert.assertEquals("BitSet in parent and subset not the same.",
          nullIndices.contains(i + shift), nulls.get(i));
    }
  }
Test if the nulls BitSet is maintained properly when we extract subset from ColumnBuffer.
E.g. suppose we have a ColumnBuffer with nulls [0, 0, 1, 0]. When we split it evenly into
two subsets, the subsets should have nulls [0, 0] and [1, 0] respectively.
serde/src/test/org/apache/hadoop/hive/serde2/thrift/TestColumnBuffer.java