One of the checks recently added to our upload system detects if there is a LOD2 with no polygons in it, and the upload is rejected in that case. This is surprisingly common in submitted mods, although there is no point in this type of configuration.
If you want to submit a mod before you have created a proper LOD2 (for middle distance and shadow) then you should put the physics LOD (which is LOD3 in a properly finished mod) into LOD2.
That is OK for unfinished mods, and you will at least have a shadow and not drive around with a vehicle that is invisible in the middle distance.
FINISHED MOD: (GOOD)
LOD1: Many triangles
LOD2: Medium triangles (e.g. around 1000, see official cars)
LOD3: Physics
UNFINISHED MOD: {OK)
LOD1: Many triangles
LOD2: Physics
WRONG:
LOD1: Many triangles
LOD2: [empty]
LOD3: Physics
EDIT: ALSO WRONG:
LOD1: Many triangles
LOD2: Many triangles, holes, ugly mess
LOD3: Physics
If you want to submit a mod before you have created a proper LOD2 (for middle distance and shadow) then you should put the physics LOD (which is LOD3 in a properly finished mod) into LOD2.
That is OK for unfinished mods, and you will at least have a shadow and not drive around with a vehicle that is invisible in the middle distance.
FINISHED MOD: (GOOD)
LOD1: Many triangles
LOD2: Medium triangles (e.g. around 1000, see official cars)
LOD3: Physics
UNFINISHED MOD: {OK)
LOD1: Many triangles
LOD2: Physics
WRONG:
LOD1: Many triangles
LOD2: [empty]
LOD3: Physics
EDIT: ALSO WRONG:
LOD1: Many triangles
LOD2: Many triangles, holes, ugly mess
LOD3: Physics