It should be possible to enable debugging without target_assembly #98

Currently when the target_assembly is not set or does not exist, Doorstop disables itself early even when debug_enabled=true. In some cases I want to debug the game, but do not want to load anything into it. Now I have to specify a dummy assembly to enable debugging.

I think debugging should work independently from assembly loading, so missing target_assembly should not prevent debugger initialization.