serial now required in 0.29.0?

My probe firmware does not provide a serial descriptor string, and pyOCD 0.28.x worked fine. 0.29.0 seems to require it now:

0000542:CRITICAL:__main__:uncaught exception: 'NoneType' object is not subscriptable
Traceback (most recent call last):
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/__main__.py", line 401, in run
    self._COMMANDS[self._args.cmd](self)
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/__main__.py", line 807, in do_commander
    PyOCDCommander(self._args, cmds).run()
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/commands/commander.py", line 65, in run
    if not self.connect():
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/commands/commander.py", line 166, in connect
    unique_id=self.args.unique_id,
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/core/helpers.py", line 138, in choose_probe
    allProbes = ConnectHelper.get_all_connected_probes(blocking=blocking, unique_id=unique_id)
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/core/helpers.py", line 83, in get_all_connected_probes
    sortedProbes = sorted(allProbes, key=lambda probe:probe.description + probe.unique_id)
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/core/helpers.py", line 83, in <lambda>
    sortedProbes = sorted(allProbes, key=lambda probe:probe.description + probe.unique_id)
  File "/home/ralphd/.local/lib/python3.6/site-packages/pyocd/probe/cmsis_dap_probe.py", line 99, in description
    board_id = self.unique_id[0:4]
TypeError: 'NoneType' object is not subscriptable

After modifying my probe firmware to provide a serial descriptor, pyocd 0.29.0 works.

Start with pyocd/core/helpers.py, where probes are sorted, and pyocd/probe/cmsis_dap_probe.py, where description accesses unique_id. Reproduce with a probe lacking a serial descriptor and trace the None value through the listing path. Done means pyocd 0.29.0 no longer crashes and can use such a probe without a serial descriptor.