Fix field not being unset if emptyContents returns false

[Dueris]

If BucketItem#emptyContents returns false, and itemLeftInHandAfterPlayerBucketEmptyEvent is set, then it wont unset it since it returned false, making it not call getEmptySuccessItem which clears the field. This simply sets the field to null if the function returns false

[WouterGritter]

It makes more sense to null itemLeftInHandAfterPlayerBucketEmptyEvent in emptyContents() instead of use(), otherwise the field is only cleared when the failure goes through use() (the field is set and can go stale in emptyContents()). This PR works now, but only because use() is currently the only caller that sees a Player.

It makes more sense to null it right before returning false in emptyContents() (after it had already been set):

if (!level.setBlock(pos, this.content.defaultFluidState().createLegacyBlock(), Block.UPDATE_ALL_IMMEDIATE)
    && !blockState.getFluidState().isSource()) {
    itemLeftInHandAfterPlayerBucketEmptyEvent = null; // Paper - Fix PlayerBucketEmptyEvent result itemstack
    return false;
}

[Dueris]

Agreed. Updated now.