Fix parsing faces that reference non-existing attributes

[lilleyse]

Fixes obj's like this where normals and uvs don't exist but they are referenced by faces.

v -1.000000 -1.000000 1.000000
v -1.000000 1.000000 1.000000
v -1.000000 -1.000000 -1.000000
v -1.000000 1.000000 -1.000000
v 1.000000 -1.000000 1.000000
v 1.000000 1.000000 1.000000
v 1.000000 -1.000000 -1.000000
v 1.000000 1.000000 -1.000000
f 1/1/1 2/2/1 4/3/1 3/4/1
f 3/5/2 4/6/2 8/7/2 7/8/2
f 7/9/3 8/10/3 6/11/3 5/12/3
f 5/13/4 6/14/4 2/15/4 1/16/4
f 3/5/5 7/17/5 5/18/5 1/16/5
f 8/19/6 4/6/6 2/15/6 6/20/6

Previously the glTF would have a uv and normal accessor full of NaNs.

[OmarShehata]

Looks good!

@lilleyse I don't have write access, but feel free to merge.