Fix a bug where whowas tries to add a user with no nick

If a user times out on registering a nick (_timeoutRegistration),
then whoWas.addUserToWhowas gets called but user.nick is None.
This causes an exception like so:

ERROR:twisted:Unhandled Error
Traceback (most recent call last):
  File "/var/env/txircd/lib/python2.7/site-packages/twisted/application/app.py", line 392, in startReactor
    self.config, oldstdout, oldstderr, self.profiler, reactor)
  File "/var/env/txircd/lib/python2.7/site-packages/twisted/application/app.py", line 313, in runReactorWithLogging
    reactor.run()
  File "/var/env/txircd/lib/python2.7/site-packages/twisted/internet/base.py", line 1192, in run
    self.mainLoop()
  File "/var/env/txircd/lib/python2.7/site-packages/twisted/internet/base.py", line 1201, in mainLoop
    self.runUntilCurrent()
--- <exception caught here> ---
  File "/var/env/txircd/lib/python2.7/site-packages/twisted/internet/base.py", line 824, in runUntilCurrent
    call.func(*call.args, **call.kw)
  File "/var/env/txircd/txircd/txircd/user.py", line 203, in _timeoutRegistration
    self.disconnect("Registration timeout")
  File "/var/env/txircd/txircd/txircd/user.py", line 193, in disconnect
    self.ircd.runActionStandard("quit", self, reason, users=self)
  File "/var/env/txircd/txircd/txircd/ircd.py", line 486, in runActionStandard
    action[0](*params, **kw)
  File "/var/env/txircd/txircd/txircd/modules/rfc/cmd_whowas.py", line 40, in addUserToWhowas
    lowerNick = ircLower(user.nick)
  File "/var/env/txircd/txircd/txircd/utils.py", line 16, in ircLower
    return string.lower().replace("[", "{").replace("]", "}").replace("\\", "|")
exceptions.AttributeError: 'NoneType' object has no attribute 'lower'