code: def test_empty(self):
      with self.assertRaises(ValueError):
          links.BinaryHierarchicalSoftmax.create_huffman_tree({})
  
imports: import copy
  import unittest
  import numpy
  import chainer
  from chainer.backends import cuda
  from chainer import gradient_check
  from chainer import links
  from chainer import testing
  from chainer.testing import attr
  from chainer.testing import condition
label_desc: Use image node_id 1 for calling the TestHuffmanTree obj's underlying member method code with example usage: obj.test_empty() without return types