Early out return 0 from BSDF methods if wo.z == 0.
Early out return 0 from BSDF methods if wo.z == 0.

This can (rarely) happen and leads to various NaN and infinity values returned from these methods.

bssrdf/pbrt-v3,mmp/pbrt-v3,mwkm/pbrt-v3,mmp/pbrt-v3,ambakshi/pbrt-v3,shihchinw/pbrt-v3,bssrdf/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,mwkm/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,ambakshi/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,bssrdf/pbrt-v3