Fixed remove_character() leaving empty indexes

[rjstebe]

When a monster kills a character, the character is removed only after update_characters(), so the characters dictionary can end up having indexes that have empty character lists, like this: {40: []}.

This can cause the condition not self.world.characters in game.done() and a similar condition in our group's terminal state checks to return False when there are no characters instead of True.

I added a couple lines to world.remove_character() to check if the removed character leaves its list empty, and if so deletes the index. Tested by printing the result of not wrld.characters in a sensed world where the monster kills the player in Variant 2 of Scenario 2, and it seems to work now.