Allow random CDS to return None, and fix weights.

game/scripts/cds.rpy

@@ -149,8 +149,9 @@ python early hide:
 
             for block, weight in blocks:
                 if n <= weight:
-                    break
+                    return block
-                else:
+                n -= weight
-                    n -= weight
 
-            return block
+            # In the rare event where weights are incorrectly assigned,
+            # return a random available block, and in any other case None.
+            return renpy.random.choice(blocks)[0] if blocks else None