Prevent from_city='none' & to_city='none' from being passed to directions

If there are more possible cities both for `from_stop` and for `to_stop`, the policy passed `from_city == 'none'` and `to_city == 'none'` on without checking, which lead to confusing prompts, such as:

    Promiňte, asi Vám nerozumím. Zastávku Budějovická v obci none neznám.
    (I am sorry, but I don't understand. I know of no stop called Budějovická in the city of none.)

This commit fixes the problem (the most important change is on line 707 of `hdc_policy.py`, the rest are rather improvements).