Optional string arguments are never NULL in Lua extension functions (FIXED)

[cfillion]

REAPER always gives a non-NULL string to 'const char*' optional arguments of extension functions, even if the caller omitted them or passed nil/0. This prevents distinguishing between omitted vs empty strings and providing a non-empty default.

int and double optional args can receive NULL as expected.

plugin_register("APIdef_Test", "bool\0const char*\0strInOptional\0\0");

bool Test(const char* strInOptional)
{
  return strInOptional != NULL;
}