I had this problem many times, if you have 2 items equiped on both of your hands (Ex: Handsaw + Axe) you can't equip a 2h item, like the bucket, but you can't also put it in your inventory if it's full, so when you have it full of water, you can't move if not by right-clicking on an item where you can use the bucket (Ex: Barrel) and you are forced to drop it in the water, losing it forever.
I think you should be able to empty it by putting it in your inventory if it's full of water, or something else, I understand that right-clicking in the ground to empty it could lend to an exploit where players keep right-clicking to move with the bucket in their cursor, but many players will lose their buckets if this is not changed somehow..