Actually fix the isWaterBottle() check

Fixes #2134

For some reason Bukkit wanted to use Potion(PotionType type, int level)
when calling .toPotion(), even though .getDataValue() is 0.

src/main/java/com/gmail/nossr50/datatypes/skills/alchemy/PotionStage.java

@@ -39,7 +39,7 @@ public enum PotionStage {
     }
 
     private static boolean isWaterBottle(AlchemyPotion input) {
-        return input.toPotion(1).getEffects().isEmpty();
+        return input.getDataValue() == 0;
     }
 
     public static PotionStage getPotionStage(AlchemyPotion alchemyPotion) {