obj2pbrt: don't emit unnamed materials
obj2pbrt: don't emit unnamed materials

shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3