fix glsl model lighting on unlit maps (normal vector was calculated as (0,0,0) as...

fix glsl model lighting on unlit maps (normal vector was calculated as (0,0,0) as there was no contribution from the map -> GLSL calls normalize() on that -> (nan,nan,nan), which apparently counts as black).
The fix replaces these zero normal vectors by (0,0,1) (which vector is actually chosen does not matter, as its contribution is scaled by zero anyway).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7248 d7cf8633-e32d-0410-b094-e92efae38249