code: def test_empty_input(self):
      data = np.array([])
      aug = nas.FrequencyMaskingAug()
      aug_data = aug.augment(data)
  
      self.assertTrue(np.array_equal(np.array([]), aug_data))
  
file_path: nlpaug/test/augmenter/spectrogram/test_frequency_masking.py
imports: import unittest
  import os
  import numpy
  from dotenv import load_dotenv
  from nlpaug.util import AudioLoader
  import nlpaug.augmenter.spectrogram
label_desc: Use image node_id 2 for calling the TestFrequencyMasking obj's underlying member method code with example usage: obj.test_empty_input() without return types