image_name can be empty

I use toaster with default settings (master) and try build poky. I don't know how toaster generate request with image_name==NoneType. But now I can't produce poky distro at master branch... It would be perfect to add autotest "if(image_name==NoneType) then error('some text')"

I suggest discus on github https://github.com/takaite/poky/issues/2