docstr: Test if an empty array is returned if no peaks are provided.
code: def test_empty(self):
      """
      Test if an empty array is returned if no peaks are provided.
      """
      widths = peak_widths([], [])[0]
      assert_(isinstance(widths, np.ndarray))
      assert_equal(widths.size, 0)
      widths = peak_widths([1, 2, 3], [])[0]
      assert_(isinstance(widths, np.ndarray))
      assert_equal(widths.size, 0)
      out = peak_widths([], [])
      for arr in out:
          assert_(isinstance(arr, np.ndarray))
          assert_equal(arr.size, 0)
  
imports: import copy
  import numpy
  from numpy.testing import assert_, assert_equal, assert_allclose, assert_array_equal
  import pytest
  from pytest import raises, warns
  from scipy.signal._peak_finding import argrelmax, argrelmin, peak_prominences, peak_widths, _unpack_condition_args, find_peaks, find_peaks_cwt, _identify_ridge_lines
  from scipy.signal._peak_finding_utils import _local_maxima_1d, PeakPropertyWarning
label_desc: Use image node_id 1 for calling the TestPeakWidths obj's underlying member method code with example usage: obj.test_empty() without return types