Allow shooting_entity on LuaEntity turrets to be set to nil

[Reika]

LuaEntity already has 'shooting_entity', but it cannot be set to nil to tell the turret not to target anything (or clear an existing target). In fact, there seems to be no way to set a turret to "No current target" at all.

[Rseding91]

Ok, I changed it for 0.17 so it can be set to nil. Now, the next tick the turret updates it will look for a new target so I'm not sure if it's that useful.

[Reika]

Ok, I changed it for 0.17 so it can be set to nil. Now, the next tick the turret updates it will look for a new target so I'm not sure if it's that useful.

Does on_tick run after this? If so, I can just clear it every tick, and so prevent a firing upon things it should not shoot.