Support players not having a played race.

Some custom maps (I'm looking at you 1v1 Obs of the Day) seem to
give players None for race type attribute in game. Map that missing
race to a python None variable instead of crashing with a KeyError.

Author: GraylinKim

sc2reader/resources.py

@@ -965,7 +965,7 @@ def load_players(self):
                 self.observers.append(player)
                 continue
 
-            player.play_race = RACE_CODES[struct[2]]
+            player.play_race = RACE_CODES.get(struct[2], None)
 
             player.is_winner = isinstance(struct[1],dict) and struct[1][0] == 0
             if player.is_winner: