Fix assert on startup when calling back on an empty string

[JonBooth78]

Fixes #5716

[Max5377]

The question is, why module_name is empty in the first place when it should normally return lua module name?

[Web-eWorks]

The module name is empty because LuaConsole.cpp is directly calling Event.Register(...) and the calling module context requested from Event.Register is a C function which has no valid filepath and therefore cannot be translated into a module name.

This technically exposes a flaw in the new Event system where C++ can only register one Lua callback handler for each event type without overwriting the previously-set handler. That's not a problem in practice (as generally speaking a C++ function shouldn't be listening for a Lua event but rather use a sigc::signal instead), but something I'll have to keep in mind for the future.