Modelled projectiles with 0 speed fail to render

Example repro.
```lua
Spring.SpawnProjectile(WeaponDefNames.vehassault_plasma.id, { -- ZK, adjust as appropriate
pos = {123, 456, 789}, -- adjust as appropriate to map
ttl = 99999,
gravity = 0, speed = {0,0,0} -- gravity needed to keep speed at 0
model = "objects3d/wep_m_ajax.s3o", -- this weapon is otherwise default yellow plasma, which works
})
```
I assume this is because the model normally points towards its speed vector. It would be fine to render it in any orientation at speed 0/0/0 as long as it renders at all. The main use case for such 0 speed projectiles is testing/development, so it's not that important, but it's very hard to tell what is going on otherwise.

Start by running the Lua Spring.SpawnProjectile reproduction with speed set to {0,0,0} and inspect the projectile rendering path. Determine how model orientation is derived when the speed vector is zero; done means the modelled projectile renders in any stable orientation without changing nonzero-speed behavior.

Written by the indexing model from the issue text.