return comparevideos stdout and fix missing device case

webrtc/tools/video_analysis_test.py

 #!/usr/bin/env python
 # Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import glob
@@ skipping 38 matching lines @@
 
     long_path1 = ('/sys/bus/usb/devices/usb1/1-1/driver/3-3/3-3.1:1.0/'
                   'video4linux/video0')
     long_path2 = ('/sys/bus/usb/devices/usb1/1-1/driver/3-2/3-2.1:1.0/'
                   'video4linux/video1')
     self.setGlobPath(long_path1, long_path2)
     long_usb_ids = ['3-3.1', '3-2.1']
     self.assertEqual(FindUsbPortForV4lDevices('video0', 'video1'), long_usb_ids)
 
 
+  def testFindUSBPortForV4lDevicesNoDevice(self):
+    noDeviceFound = ('')
+    V4lDevice = ('/sys/bus/usb/devices/usb1/1-1/driver/3-2/3-2.1:1.0/'
+                  'video4linux/video1')
+    self.setGlobPath(noDeviceFound, V4lDevice)
+    empty_list = []
+    self.assertEqual(FindUsbPortForV4lDevices('video0', 'video1'), empty_list)
+
+
 if __name__ == "__main__":
   unittest.main()