Update app.py

app.py

@@ -8,12 +8,12 @@ from chess_puzzler.tagger import cook
 def find_puzzles(pgn_text):
     game = chess.pgn.read_game(StringIO(pgn_text))
     if not game:
-        return "Invalid PGN"
+        return None
     engine = open_engine("/usr/games/stockfish", threads=2)
     try:
         puzzle = Generator(engine).analyze_game(game, tier=10)
         if not puzzle:
-            return "No puzzle found"
+            return None
         puzzle.tags = cook(puzzle)
         return puzzle.to_dict()
     finally: