function testDeferrable_ImplicitTx_IgnoreNull() {
  var childRow = childTable.createRow({
    id: 'childId',
    parentId: null,
    name: 'childName'
  });

  return db.insert().into(childTable).values([childRow]).exec().then(
      function() {
        return lf.testing.util.selectAll(global, childTable);
      }).then(
      function(results) {
        assertEquals(1, results.length);
        assertNull(results[0].payload()['parentId']);
      });
}
Tests that a child column value of null, does not trigger a foreign key
constraint violation, instead it is ignored.
@return {!IThenable}