docstr: Test firwin2 when window=None.
docstr_tok: ["Test","firwin2","when","window=None","."]
code: def test04(self, xp, scp):
      """Test firwin2 when window=None."""
      ntaps = 5
      # Ideal lowpass: gain is 1 on [0,0.5], and 0 on [0.5, 1.0]
      freq = xp.asarray([0.0, 0.5, 0.5, 1.0])
      gain = xp.asarray([1.0, 1.0, 0.0, 0.0])
      taps = scp.signal.firwin2(
          ntaps, freq, gain, window=None, nfreqs=8193
      )
      return taps
  
imports: import cupy
  import cupyx.scipy.signal
  from cupy import testing
  import pytest
  from pytest import raises
label_desc: Use image node_id 5 for calling the TestFirwin2 obj's underlying member method code with example usage: obj.test04(xp, scp) and returns: taps