fix queen side castling issue

Unable to provide queen side castling for `r1bq1rk1/pp2npbp/2np2p1/2p5/4P2P/2NPB1P1/PPP1QPB1/R3K2R w KQq - 0 1` FEN.

lib/src/game/game.dart

@@ -468,7 +468,7 @@ class Game {
               continue;
             } // king starting on target
 
-            if (j != numMidSqs && board[midSq].isNotEmpty) {
+            if (j != numMidSqs && from != midSq && board[midSq].isNotEmpty) {
               // squares between to and from must be empty
               valid = false;
               break;