Bug: `autodump` and `gui/autodump` dumping in midair causes items to disappear

[Bumber64]

Autodumping in midair causes items to be placed without the on_ground flag, making them invisible. The plugin code is supposed to be turning them into projectiles, but that's failing for whatever reason.

[Bumber64]

I see this was brought up in #5427, which was only partially fixed with #5432. (The changelog incorrectly notes this as a fix to gui/teleport, but that's for units.)