d3d12: Always compile some shader variant

Not compiling a shader is not an option, because we need to assign a current shader when a new shader selector is created, otherwise evaluating the shader key at link time will result in dereferencing a null pointer. Since some shaders can't be successfully compiled and need lowering, and this lowering is dependent on the state, we may compile junk shaders here, but with the current design of the shader selector that is a performance hit we just have to accept for now.

There may be a better option to deal with this, but this would need some redesign of the shader selector which would translate into quite some time with a broken master.

Fixes: c26baf25 d3d12: Never compile the initial shader