Fix OBJExporter without normals

[fernandojsg]

Related: #13400 (comment)

[fernandojsg]

I just changed it so if there's just a vertex without uvs and normals it will just export the ID itself without any backslash.

[fernandojsg]

Example exporting a cube to OBJ from the editor:

uvs and normals:

o Box
v 0.5 0.5 0.5
v 0.5 0.5 -0.5
v 0.5 -0.5 0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 -0.5
v -0.5 0.5 0.5
v -0.5 -0.5 -0.5
v -0.5 -0.5 0.5
v -0.5 0.5 -0.5
v 0.5 0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v -0.5 -0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v 0.5 0.5 -0.5
v -0.5 0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 -0.5 -0.5
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vn 1 0 0
vn 1 0 0
vn 1 0 0
vn 1 0 0
vn -1 0 0
vn -1 0 0
vn -1 0 0
vn -1 0 0
vn 0 1 0
vn 0 1 0
vn 0 1 0
vn 0 1 0
vn 0 -1 0
vn 0 -1 0
vn 0 -1 0
vn 0 -1 0
vn 0 0 1
vn 0 0 1
vn 0 0 1
vn 0 0 1
vn 0 0 -1
vn 0 0 -1
vn 0 0 -1
vn 0 0 -1
f 1/1/1 3/3/3 2/2/2
f 3/3/3 4/4/4 2/2/2
f 5/5/5 7/7/7 6/6/6
f 7/7/7 8/8/8 6/6/6
f 9/9/9 11/11/11 10/10/10
f 11/11/11 12/12/12 10/10/10
f 13/13/13 15/15/15 14/14/14
f 15/15/15 16/16/16 14/14/14
f 17/17/17 19/19/19 18/18/18
f 19/19/19 20/20/20 18/18/18
f 21/21/21 23/23/23 22/22/22
f 23/23/23 24/24/24 22/22/22

just uvs

o Box
v 0.5 0.5 0.5
v 0.5 0.5 -0.5
v 0.5 -0.5 0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 -0.5
v -0.5 0.5 0.5
v -0.5 -0.5 -0.5
v -0.5 -0.5 0.5
v -0.5 0.5 -0.5
v 0.5 0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v -0.5 -0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v 0.5 0.5 -0.5
v -0.5 0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 -0.5 -0.5
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
vt 0 1
vt 1 1
vt 0 0
vt 1 0
f 1/1 3/3 2/2
f 3/3 4/4 2/2
f 5/5 7/7 6/6
f 7/7 8/8 6/6
f 9/9 11/11 10/10
f 11/11 12/12 10/10
f 13/13 15/15 14/14
f 15/15 16/16 14/14
f 17/17 19/19 18/18
f 19/19 20/20 18/18
f 21/21 23/23 22/22
f 23/23 24/24 22/22

just normals

o Box
v 0.5 0.5 0.5
v 0.5 0.5 -0.5
v 0.5 -0.5 0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 -0.5
v -0.5 0.5 0.5
v -0.5 -0.5 -0.5
v -0.5 -0.5 0.5
v -0.5 0.5 -0.5
v 0.5 0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v -0.5 -0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v 0.5 0.5 -0.5
v -0.5 0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 -0.5 -0.5
vn 1 0 0
vn 1 0 0
vn 1 0 0
vn 1 0 0
vn -1 0 0
vn -1 0 0
vn -1 0 0
vn -1 0 0
vn 0 1 0
vn 0 1 0
vn 0 1 0
vn 0 1 0
vn 0 -1 0
vn 0 -1 0
vn 0 -1 0
vn 0 -1 0
vn 0 0 1
vn 0 0 1
vn 0 0 1
vn 0 0 1
vn 0 0 -1
vn 0 0 -1
vn 0 0 -1
vn 0 0 -1
f 1//1 3//3 2//2
f 3//3 4//4 2//2
f 5//5 7//7 6//6
f 7//7 8//8 6//6
f 9//9 11//11 10//10
f 11//11 12//12 10//10
f 13//13 15//15 14//14
f 15//15 16//16 14//14
f 17//17 19//19 18//18
f 19//19 20//20 18//18
f 21//21 23//23 22//22
f 23//23 24//24 22//22

just vertex

o Box
v 0.5 0.5 0.5
v 0.5 0.5 -0.5
v 0.5 -0.5 0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 -0.5
v -0.5 0.5 0.5
v -0.5 -0.5 -0.5
v -0.5 -0.5 0.5
v -0.5 0.5 -0.5
v 0.5 0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v -0.5 -0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 0.5 0.5
v 0.5 0.5 0.5
v -0.5 -0.5 0.5
v 0.5 -0.5 0.5
v 0.5 0.5 -0.5
v -0.5 0.5 -0.5
v 0.5 -0.5 -0.5
v -0.5 -0.5 -0.5
f 1 3 2
f 3 4 2
f 5 7 6
f 7 8 6
f 9 11 10
f 11 12 10
f 13 15 14
f 15 16 14
f 17 19 18
f 19 20 18
f 21 23 22
f 23 24 22

[mrdoob]

Thanks!