How can I switch between direct lighting and light probes in a shader?

We have created a set of custom vertex and fragment shaders. They use the regular ShadeSH9 function to sample the light probes.

Sadly, this disables any direct lighting influence on the rendered objects. How can I have that back and, maybe even more importantly, detect and switch in the shader if a user has enabled/disabled light probes for the renderer.

Thanks!