Ancient API break in IScriptEnvironment::Invoke()

[myrsloik]

In Avisynth 2.5.x arg_names that are a pointer to an empty string ("") get treated the same as passing NULL. This behavior changed from 2.6 onward and actually breaks old yatta builds. Just thought I'd mention it since it took me a while to figure it out. In case you're wondering why yatta does it it's because of a typo in the code.