"Debt Report" should work even if a member is missing a first payment

[facundobatista]

Currently getting this when listing the report:

TypeError: '>' not supported between instances of 'NoneType' and 'int'
  File "django/core/handlers/exception.py", line 35, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 128, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 126, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "members/views.py", line 60, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "django/contrib/auth/mixins.py", line 52, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "django/views/generic/base.py", line 89, in dispatch
    return handler(request, *args, **kwargs)
  File "members/views.py", line 174, in get
    debt = logic.get_debt_state(member, limit_year, limit_month)
  File "members/logic.py", line 142, in get_debt_state
    elif member.first_payment_year > limit_year: