Fix empty slug problem causeing 500 error when project name is all Chinese

When project name is all Chinese, django's slugify() will return empty string. It will cause 500 error like this:

	Internal Server Error: /account/teams/tech/projects/new/
	Traceback (most recent call last):
	  File "/home/env/sentry/lib/python2.6/site-packages/Django-1.4.5-py2.6.egg/django/core/handlers/base.py", line 111, in get_response
		response = callback(request, *callback_args, **callback_kwargs)
	  File "/home/env/sentry/lib/python2.6/site-packages/Django-1.4.5-py2.6.egg/django/utils/decorators.py", line 91, in _wrapped_view
		response = view_func(request, *args, **kwargs)
	  File "/home/env/sentry/lib/python2.6/site-packages/sentry-5.4.4-py2.6.egg/sentry/web/decorators.py", line 110, in _wrapped
		return func(request, *args, **kwargs)
	  File "/home/env/sentry/lib/python2.6/site-packages/sentry-5.4.4-py2.6.egg/sentry/web/frontend/teams.py", line 455, in create_new_team_project
		return HttpResponseRedirect(reverse('sentry-docs-client', args=[project.team.slug, project.slug, project.platform]))
	  File "/home/env/sentry/lib/python2.6/site-packages/Django-1.4.5-py2.6.egg/django/core/urlresolvers.py", line 476, in reverse
		return iri_to_uri(resolver._reverse_with_prefix(view, prefix, *args, **kwargs))
	  File "/home/env/sentry/lib/python2.6/site-packages/Django-1.4.5-py2.6.egg/django/core/urlresolvers.py", line 396, in _reverse_with_prefix
		"arguments '%s' not found." % (lookup_view_s, args, kwargs))
	NoReverseMatch: Reverse for 'sentry-docs-client' with arguments '(u'tech', u'', u'flask')' and keyword arguments '{}' not found.

So a default slug should be provided when base_slug is empty.