Fix for when you have no lights
Fix: Allow Value node to be used for Strength
Nicer support for custom Strength node

This means that is kind of actually works ;) Though I'm still unhappy that it's needed at all.
An automatic way of figuring which node should be used to strength would be best (as this is already done for Color), but there are just too many possible problems with that for me to solve at the moment. For a later release perhaps.
Poll for render engine