Skip LLVM JIT for empty shaders.
Skip LLVM JIT for empty shaders.

A good optimization in any case, but some of the new code triggered
assertions in LLVM that only seemed to happen for empty shaders.

gzorin/OpenShadingLanguage,gzorin/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,lgritz/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,brechtvl/OpenShadingLanguage,brechtvl/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,aconty/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,gzorin/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,gzorin/OpenShadingLanguage,brechtvl/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,aconty/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,gzorin/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,lgritz/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,aconty/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,fpsunflower/OpenShadingLanguage