The current spigot PlayerBucketFillEvent is called even if the player does not actually fill up their bucket.
You may reproduce this by right clicking a waterloggable block, such as stairs, that are not waterlogged at the time.
A possible easy solution would be to simply ensure that the `dummyFluid` fetched prior to the event call is not empty. This would ensure that only non-air item results (e.g. water buckets) trigger the event.