[tannerweb] Empty last session results to server error

[afeena]

Error handling request
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/aiohttp/web_protocol.py", line 418, in start
    resp = await task
  File "/usr/lib/python3.6/site-packages/aiohttp/web_app.py", line 458, in _handle
    resp = await handler(request)
  File "/usr/lib/python3.6/site-packages/aiohttp_jinja2/__init__.py", line 99, in wrapped
    app_key=app_key, encoding=encoding)
  File "/usr/lib/python3.6/site-packages/aiohttp_jinja2/__init__.py", line 71, in render_template
    text = render_string(template_name, request, context, app_key=app_key)
  File "/usr/lib/python3.6/site-packages/aiohttp_jinja2/__init__.py", line 62, in render_string
    text = template.render(context)
  File "/usr/lib/python3.6/site-packages/jinja2/asyncsupport.py", line 76, in render
    return original_render(self, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/usr/lib/python3.6/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python3.6/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "tanner/web/templates/session.html", line 1, in top-level template code
    {% extends "base.html" %}
  File "tanner/web/templates/base.html", line 28, in top-level template code
    {% block content%}
  File "tanner/web/templates/session.html", line 67, in block "content"
    {% for attack, v in session.attack_count.items() %}
  File "/usr/lib/python3.6/site-packages/jinja2/environment.py", line 430, in getattr
    return getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'None' has no attribute 'attack_count'