It's allowed in OBJ file format to simply omit texture attribute. Thus, f 1//1 2//1 3//1
is a valid triangle, just only with positions and normals. You should change your parser to accept that (which shouldn't be very hard).
Wikipedia article about OBJ explains that rather well:
Vertex/normal
As texture coordinates are optional, one can define geometry without them, but one must put two slashes after the vertex index before putting the normal index:
f v1//vn1 v2//vn2 v3//vn3
Also keep in mind that you don't have to specify normals either, and a face can look like:
f 1/1 2/2 3/3